Halogen bonding and chalcogen bonding mediated sensing

Sigma–hole interactions, in particular halogen bonding (XB) and chalcogen bonding (ChB), have become indispensable tools in supramolecular chemistry, with wide-ranging applications in crystal engineering, catalysis and materials chemistry as well as anion recognition, transport and sensing. The latter has very rapidly developed in recent years and is becoming a mature research area in its own right. This can be attributed to the numerous advantages sigma–hole interactions imbue in sensor design, in particular high degrees of selectivity, sensitivity and the capability for sensing in aqueous media. Herein, we provide the first detailed overview of all developments in the field of XB and ChB mediated sensing, in particular the detection of anions but also neutral (gaseous) Lewis bases. This includes a wide range of optical colorimetric and luminescent sensors as well as an array of electrochemical sensors, most notably redox-active host systems. In addition, we discuss a range of other sensor designs, including capacitive sensors and chemiresistors, and provide a detailed overview and outlook for future fundamental developments in the field. Importantly the sensing concepts and methodologies described herein for the XB and ChB mediated sensing of anions, are generically applicable for the development of supramolecular receptors and sensors in general, including those for cations and neutral molecules employing a wide array of non-covalent interactions. As such we believe this review to be a useful guide to both the supramolecular and general chemistry community with interests in the fields of host–guest recognition and small molecule sensing. Moreover, we also highlight the need for a broader integration of supramolecular chemistry, analytical chemistry, synthetic chemistry and materials science in the development of the next generation of potent sensors.


Introduction
Halogen bonding (XB) and chalcogen bonding (ChB), the noncovalent interactions between a positively charged region on a polarised, electron decient halogen or chalcogen atom (the s-hole) and a Lewis base, have emerged as powerful and potent additions to the supramolecular chemistry tool-box, being increasingly exploited in catalysis, 1-6 crystal engineering 7-12 and most notably molecular recognition. [13][14][15][16][17][18][19][20][21] Stimulated by the importance of negatively charged species in a plethora of biological, industrial and environmental spheres, the XB-mediated recognition of anions, in particular, has signicantly advanced in recent years. 14,16,[22][23][24] Due to their comparatively lower chargedensity, stronger hydration as well as pH-dependence, the selective recognition of anions is signicantly more challenging than cations, especially in aqueous environments. [25][26][27] XB and ChB are ideally suited to address this challenge, as they typically imbue both enhanced selectivity and binding strength in comparison to hydrogen bonding (HB) analogues. This can be attributed to a variety of factors including a stricter adherence to a 180 binding geometry, lower solvent dependency, larger hydrophobicity and improved electronic tuneability. 14,16,28,29 These combined advantages have facilitated sigma-hole mediated anion recognition in aqueous media, importantly including pure water. [30][31][32] As a result, increasing attention is being directed at the application of this capability in transmembrane anion transport, [33][34][35][36][37] ion extraction and remediation 38,39 as well as anion sensing. [40][41][42] The latter is highly relevant in many real-world scenarios, in particular environmental and healthcare monitoring. The reversible nature of non-covalent binding interactions in supramolecular host-guest systems is ideally suited for repeat and long-term sensing applications. To this end, the generation of anion sensors by integration of suitable optical or electrochemical reporter groups into hydrogen bond donor anion receptor structural frameworks has received enormous attention over the past few decades. 27,[40][41][42][43][44][45] In contrast, XB and ChB mediated anion sensing is a relatively new phenomenon, oen demonstrating enhanced selectivity, binding strength and enhanced signal transducing capabilities in comparison to HB based sensing analogues.
Herein, we provide an overview of this eld with a particular focus on the analytical performance of XB and ChB anion sensors. Where possible we contrast these sensors with analogous HB systems, highlighting the origins of enhanced XB and ChB sensing performances in the context of fundamental hostguest recognition principles.

General overview and scope of the review
To date, ion-selective electrodes (ISEs) are the main employed anion sensors. Simple and cheap, they nevertheless oen fail certain sensing criteria, as they possess thermodynamically limited sensitivities and require frequent (re)calibration. [46][47][48][49] In order to satisfy a broad(er) range of application criteria, improved or complementary sensing approaches are thus highly desired. This has spurred on the development of a large range of alternative optical or electrochemical supramolecular anion sensors, [40][41][42][43]50 wherein the use of s-hole interactions is increasingly recognised as a particularly potent means to achieve higher degrees of selectivity and sensitivity as well as guest recognition and sensing in aqueous media.
This review gives a broad state-of-the art overview of all types of XB and ChB based sensors for anions, and also neutral Lewis bases and gases with a focus on their sensing performance in the context of sigma-hole recognition. Furthermore, we highlight how XB and ChB sensor systems contribute to fundamental aspects of supramolecular interactions as well as how they can aid in future developments in the elds of supramolecular host-guest chemistry and (ion) sensing in general.
The review begins with an introduction of the intrinsic properties of s-hole interactions (Section 2.2) and their inuence on relevant sensor parameters (Section 2.3). This is followed by a discussion of relevant examples of XB and ChB colorimetric and luminescent sensors (Section 3) as well as redox-active sensors (Section 4.1). The vast majority of these sensors are constructed by covalent appendage of reporter groups to XB or ChB receptors, whose optical or electrochemical properties are reversibly modulated in the presence of the bound guest analyte, as schematically shown in Fig. 1.
Nevertheless, a range of sensors omitting any dedicated reporter groups have been developed, including capacitive or chemiresistive sensors as discussed in Sections 4.2 (other electrochemical sensors) and 5 (other sensors). Finally, we provide an overview of future developments in the rapidly advancing eld of sigma-hole mediated sensing and (an)ion sensing more generally (Section 6). Fig. 1 Schematic depiction of the most common supramolecular host-guest sensing approach based on the guest-binding induced modulation of the optical or electrochemical properties of a reporter group appended to a synthetic receptor. Shown here is the halogen bonding (XB) or chalcogen bonding (ChB)-mediated recognition of an anion as an archetypical Lewis base and its subsequent detection via various readouts.

Brief introduction to sigma-hole interactions
Appending a group 14-17 atom (X) with a substituent of higher electron-withdrawing capability (R) induces an anisotropic distribution of electron density such that a region of positive electrostatic potential is formed along the elongation of the R-X s-bond (Fig. 2). This so-called s-hole can then non-covalently interact with Lewis bases (LBs), forming halogen (XB), chalcogen (ChB), pnictogen (PnB) or tetrel bonds (TrB), for X ¼ group 17, 16, 15 and 14, respectively. 14 The strength of these shole interactions is, of course, not only dependent on the nature and Lewis basicity of the acceptor, but is also highly tuneable via modication of the s-hole donor scaffold (for examples see Fig. 2). Specically, "deeper", that is more electropositive, sholes are formed when the appended substituent is more electron-withdrawing. Similarly, larger, more polarisable and less electronegative donor atoms form more potent s-holes, such that XB (and ChB) strength generally increases for the analogues descending the respective main group, i.e. I > Br > Cl and Te > Se > S. 14 † Notably for XB the s-hole is highly localised along the elongation of the R-X s-bond, resulting in highly directional, linear non-covalent bond formation with Lewis bases, oen exceeding 170 for the R-X/LB bond angle. These effects are primarily rationalised within an electrostatic bonding framework, that is based on the concept of the s-hole. While conceptually useful and straight-forward, this description is not always sufficient to describe all sigma-hole properties. Specically, a range of other factors, including polarisation, dispersion forces, hydrophobic effects and orbital interactions need to be considered in an accurate description of the nature of s-hole bonding. 14 This notably includes (partial) charge transfer from the Lewis base into the s* orbital of the donor. 51 This interaction between an occupied orbital of the Lewis base guest and an empty donor orbital is not only highly directional, but can also be a substantial driving force for sigma-hole bonding, giving rise to a signicant degree of covalent character. 29,[52][53][54] This has also been experimentally observed for ChB interactions 29 as well as halide-XB complexes. 55 Recently, pcovalency was also shown to play a role in XB bond formation with halides. 56 These observations may account for specic selectivity preferences, an improved sensor sensitivity as well as a generally lowered solvent dependence of s-hole interactions in comparison to HB. 29,57,58 For more in-depth discussions on the nature of s-hole interactions the interested reader is referred to recent reviews. 14,51,[59][60][61] Of further note is that, depending on their hybridisation, ChB donors can form one (sp 2 ) or two sigma holes (sp 3 ). The "additional" substituent of sp 3 hybridised ChB donors (in comparison to only one substituent on typical XB or HB donors), hereby presents a potent means of further tuning the s-hole strength, or providing additional functionalities, such as optical or electrochemical reporter groups. 62

Relevant sensor parameters
As alluded to above, an important goal in (supramolecular) sensor development is the capability of sensing in aqueous media. Their lower solvent dependence renders s-hole interactions particularly potent in this regard. Similarly, the inherent characteristics of s-hole interactions are highly relevant, and oen benecial, in addressing some of the other main goals in sensing and directly impacts most of the relevant sensing parameters as described in the following (Fig. 3).
2.3.1 Selectivity. Achieving a high degree of selectivity is, at least from a supramolecular chemistry point of view, one of the most important goals in host-guest chemistry, and, by extension, the development of derived sensors. In this context, shole interactions are, by virtue of their stricter geometric bonding preferences, ideal tools to address this challenge. As will be apparent from many examples in the following sections,  XB and ChB mediated recognition and sensing is, in comparison to HB, oen not only associated with contrasting selectivity patterns but also with an overall enhanced selectivity for a specic Lewis base (anion).
However, it also should be noted that highly selective recognition is not an absolute requirement for the construction of potent sensors. Firstly, depending on the specic application scenario, as well as the desired sensitivity, the co(existence) of certain levels of competing anions may be tolerated, in particular if their concentrations are low or their binding sufficiently weaker so as to not signicantly compete with recognition and sensing of the target analyte.
In addition, the requirement for a high degree of selectivity of any one sensor may be overcome by employing an array of multiple different receptive sensor units such that a broader data set is obtained. From this, the desired sensory parameters, that is the concentration of one or multiple species, can then be obtained by, for example, deep-learning algorithms or more classical approaches such as linear discriminate analysis (LDA) or principal component analysis (PCA). [63][64][65][66] This is an established approach to sense various analytes, including ions, 63-65,67-69 but remains thus far unexplored in the context of sigma-hole mediated sensing. Sigma-hole interactions can offer much in this regard, as their typically contrasting selectivity and sensing patterns can complement that of more established sensors based on HB or electrostatic interactions.
2.3.2 Sensitivity and limit of detection. An important characteristic of any sensor is its analytical performance in terms of the linear/dynamic concentration range over which the analyte can be detected and its sensitivity, that is the slope of the calibration curve, i.e. how sensitive the sensor's signal is to a change in analyte concentration. ‡ A related, crucial parameter is the limit of detection (LOD), the lowest analyte concentration that can be reliably distinguished from the background noise. The LOD can be determined in different ways, but is commonly assessed as LOD ¼ 3 Â SD/S where SD represents the standard deviation of the response/blank and S the slope of the calibration curve (sensitivity). In many applications a low LOD is desired, which can be achieved by either lowering SD or increasing the sensitivity. The former is related to the methodology/instrumentation itself while the latter can be improved by chemical design of the sensor. Two considerations are of importance when aiming to improve probe sensitivity; stronger binding and a larger magnitude signal perturbation response from the reporter group upon binding, both of which are typically observed in sigma-hole based sensors (Fig. 4). Specically, a larger binding constant will induce a higher degree of complexation, and associated signal change. Similarly, higher sensitivity will be achieved when the signal difference between the free host sensor and the host-guest complex is larger, i.e. when analyte binding induces larger electronic perturbations to the host, as is oen the case in XB and ChB mediated sensors (see Fig. 4, larger Dsignal).

Device integration.
This oen-overlooked requirement perhaps presents the most important, yet largely unaddressed challenge in translating ion sensors from the lab to real-life applications. In the development of real-life relevant sensors, a plethora of additional factors, including sample preparation, signal readout, stability, shelf-life, reusability, cost, simplicity, user-friendliness and many others have to be considered. While most of these are not directly relevant to the supramolecular design of ion sensors, some requirements, in particular device integration, are increasingly pertinent and should be considered in molecular design. Specically, to date, the vast majority of reported ion sensors, both optical and electrochemical, operate in homogeneous solution, a setting in which the most important advantage of the host-guest sensing approach, its reversibility, cannot be easily exploited. This is because in homogeneous solution, the recovery and re-use of the supramolecular host probe is typically not feasible, such that the host-guest approach has no inherent merit over irreversible, reaction-based chemodosimeters. In order to leverage the reversibility of the non-covalent interaction with the analyte, the supramolecular host probe needs to be integrated into condensed matter such as interfaces, gels, polymers, metalorganic frameworks (MOFs) or (nano)particles. This enables facile continuous sensing by owing of the sample solution over/through the aforementioned systems, 71,72 but typically requires integration of anchor groups into the host scaffold. In this context sp 3 -hybridised ChB donor motifs are uniquely potent structural scaffolds, as the additional substituent allows for facile integration of (added) functionalities such as reporter Fig. 4 Simulated response isotherms of a generic non-covalent hostguest sensor based on the reporter group approach. The binding isotherms are simulated according to a simple 1 : 1 host-guest stoichiometric binding model with [H] ¼ 100 mM and an initial free host signal of S H ¼ 1. 70 Depicted are three exemplary cases: black line: strong binding (K ¼ 10 4 M À1 ) and a large signal change of the hostguest complex (S HG ¼ 10, DS ¼ 9) as typically encountered in sigmahole based sensors. The blue and red lines represent typical examples of less potent HB sensors in which either binding is weaker (blue line, K ¼ 10 3 M À1 ) or in which the signal magnitude is smaller (red line, S HG ¼ 5, DS ¼ 4). This highlights the different mechanisms by which sigmahole based sensors typically outperform related HB analogues in terms of sensitivity. Inset: magnification at low guest concentrations. or anchor motifs. However, this capability remains largely unexplored. 62,73 3. Optical sensors

Colorimetric sensors
Perhaps the simplest example of a colour dependence in which XB plays a pivotal role is that of dissolved iodine; violet in the gasphase as well as in non-polar solvents, while brownish in solvents of higher polarity. Over a century ago in 1903, Lachmann attributed this observation to formation of solvent$I 2 adducts, 74 whose wavelength of absorption is increasingly hypsochromically shied for solvents with higher Lewis basicity. 15,75 While this, and many of the following examples, can scarcely be considered useful real-life sensors, the use of UV-vis spectroscopy has, due to its simplicity and ubiquity, received enormous attention in the study of host-guest interactions, including sigma-hole bonding and sensing. 76 In fact, many examples of XB-based anion receptors have been shown to undergo changes in absorbance upon anion recognition. This includes, for example, acyclic bimetallic rhenium(I)containing XB iodotriazole receptors, 77 acyclic and macrocyclic zinc(II)-porphyrin iodotriazole XB hosts, 78,79 XB iodopyridinium helicates 80 and XB/HB iodoperuoroaryl urea receptors. 81 Recently, the groups of Huber and Rosokha specically employed UV-vis spectroscopy to prove the formation of "antielectrostatic" XB between halide anions and an anionic iodocyclopropenylium XB host in solution (Fig. 5). 82 Similarly, UVvis spectroscopy was employed to investigate guest binding in a range of ChB receptors. 36 However, it must be noted that in the majority of these examples no systematic sensing studies were carried out and that the absorbance changes in most systems are only small; very few simple receptors undergo signicant (naked-eye visible) changes. 78,87,88 A powerful, but comparably rare strategy to induce large scale, naked-eye colorimetric changes upon anion recognition is the use of receptor co-conformational changes in mechanically interlocked molecules (MIMs) such as rotaxanes or catenanes upon guest binding. 41 XB-mediated anion sensing via this strategy has been investigated by the Beer group in a range of [2] and [3]rotaxane shuttles. [89][90][91] For instance, the bistable rotaxanes 1.XB/HB were developed as colorimetric anion sensors, undergoing halide bindinginduced co-conformational changes and a concomitant colour change (Fig. 6). 90 Specically, in the absence of a coordinating anion guest, the electron-rich hydroquinone-containing macrocycle resides preferentially at the NDI station of the axle, resulting in an orange colour arising from donor-acceptor charge-transfer interactions. Upon addition of Cl À or I À , convergent anion binding from the macrocycle's isophthalamide HB donors and the axle's (iodo)triazolium XB/HB donors induces a shuttling of the macrocycle to the triazolium  station, thereby disrupting the donor-acceptor charge-transfer interactions resulting in a loss of colour.
Importantly, in the presence of 1 equiv. of halide anions in CDCl 3 both 1.XB and 1.HB displayed an (almost) quantitative occupation of the triazolium station of at least 92% and 100%, respectively (Table 1). However, in the absence of coordinating anions, only the XB shuttle displayed a preferential occupation of the NDI station (62%), while in the HB system the macrocycle preferentially resided at the triazolium station (76%). This shows that the XB rotaxane is a superior shuttle, with larger changes in station occupancy upon anion recognition. This enhanced shuttling, and thus sensing, capability of 1.XB was also conrmed in more polar solvent systems. As shown in Table 1, in more competitive CDCl 3 /MeOD 4 : 1 and CDCl 3 / MeOD 1 : 1 both rotaxanes displayed reduced shuttling capabilities in the presence of halides, albeit with a generally superior performance of 1.XB. Notably in these competitive solvents the XB system showed a much more signicant shuttling performance in the presence of I À with an impressive 30% station occupancy change in CDCl 3 /MeOD 1 : 1, while 1.HB displayed a 10-fold worse shuttling behaviour (3% change, Table 1).
Building on these results, a more elaborate, structurally related XB/HB four station [3]rotaxane containing two triazolium and two NDI stations, as well as two hydroquinoneisophthalamide macrocycles was developed. 89 Upon addition of Cl À or NO 3 À in CDCl 3 , the macrocycles undergo a concerted pincer-type shuttling motion from the peripheral NDI stations to the central triazolium stations and, as in the previous example, induce a colour change from orange to colourless. Binding of the smaller halide anion proceeds via 1 : 2 hostguest stoichiometric binding, while the larger, trigonal planar NO 3 À binds strongly with a 1 : 1 stoichiometry, bridging the axle and both macrocycles. Of further note is not only the expectedly enhanced XB recognition performance of the XB [3]rotaxane, but also a rare NO 3 À selectivity over Cl À and a range of other oxoanions. In a more recent example, Klein et al. prepared the bistable [2]rotaxane shuttle 2.XB for anion and pH dependent molecular motion and sensing (Fig. 7). 91 In analogy to the previous examples, the hydroquinone-isophthalamide-containing macrocycle of the neutral, unprotonated rotaxanes preferentially resides on the axle electron decient naphthalimide motif, resulting in a yellow colouration. In CDCl 3 neither protonation of the benzimidazole nor presence of Cl À alone induced any shuttling of the macrocycle. Only in the presence of both acid and anion was macrocycle translocation to the benzimidazolium-iodotriazole anion binding station observed, resulting in loss of colour as well as uorescence emission increase. The rotaxane shuttle thus behaves as a molecular logic AND gate, requiring both a coordinating anion as well as anion binding enhancement via benzimidazole-protonation to function.

Luminescent sensors
In an effort to provide a more sensitive and useful sensor readout in comparison to the colorimetric sensors discussed above, the development of luminescent sigma-hole-based probes has gained signicant attention in the last decade. To this end, a diverse range of XB, and to a much lesser extent ChB, acyclic, macrocyclic and interlocked receptor architectures have been endowed with various organic and transition-metal based luminescent motifs, providing a simple and highly sensitive means of sensing of various anions. [41][42][43] 3.2.1 Halogen bonding luminescent sensors. One of the rst examples of a potent XB uorescent anion sensor system  92 The macrocyclic haloimidazolium hosts 3a-c.XB (Fig. 8) were investigated as anion receptors and sensors in the highly competitive CH 3 OH/H 2 O 9 : 1 solvent system, wherein 1 H NMR studies revealed strong 1 : 1 host-guest stoichiometric binding of Br À and I À to the synisomers of the bromo-and iodo-imidazolium hosts 3b.XB and 3c.XB (K > 10 000 M À1 for 3b.XB).
In contrast, a large range of oxoanions (H 2 PO 4 À , NO 3 À , SO 4 2À , AcO À , BzO À ) as well as F À and Cl À did not bind to these hosts. Similarly, neither the weaker XB Cl-donor derivative 3a.XB nor the anti conformer of 3c.XB (capable of only forming one XB-anion interaction) bound any of the tested guests, while binding of the three halides Cl À , Br À and I À to the HB host 3.HB was very weak (<85 M À1 ). Fluorescence sensing studies mirrored these trends; neither 3.HB nor 3a.XB responded to any anions, while signicant enhancement of the naphthalene emission, in particular of the initially weaker low-energy band, was observed for both the bromo-and syn iodo-imidazolium hosts in the presence of Br À and I À (Fig. 9). Interestingly, 3b.XB displayed more signicant emission enhancements of up to 5.8Â in the presence of I À (K ¼ 63 100 M À1 ) than Br À (2Â, K ¼ 2880 M À1 ), while the iodoimidazolium receptor displayed preferential Br À binding and enhancements (6.4Â, K ¼ 95 500 M À1 ), with weaker I À binding (1.6Â, K ¼ 3710 M À1 ). This highlights the enormous potency of XB for tuneable, highly selective halide sensing in aqueous media.
Building on this motif, the groups of Caballero and Molina developed related, anthracene appended acyclic (bromo)imidazolium receptors 4.XB/HB as well as the mixed XB and HB receptor 4.XB+HB. 93,94 In ACN 4.XB displayed no uorescence changes in the presence of Cl À , Br À , I À and various oxoanions, most notably HSO 4 À , AcO À and BzO À , whereas F À and SO 4 2À induced signicant uorescence quenching. 93  , with no appearance of excimer emission. The mixed 4.XB+HB receptor displays response patterns that are generally identical to those of the XB probe, with an additional moderate quenching response towards AcO À . 94 In a later study a related tripodal bromoimidazolium anthracene receptor displayed similar selective detection of H 2 PO 4 À via the same excimer response mechanism. 95 The same group also developed a tetra bromoimidazole-tetraphenylethylene as an ion-pair receptor, capable of selective emission turn-on sensing of HSO 4 À in the presence of co-bound Zn 2+ in ACN. 96 In analogy to many of the other sensors described in the other sections of this review, the ubiquitous 5-iodo-1,2,3triazole motif has been exploited in a large range of uorescent XB anion sensors. For example, Zapata et al. developed a range of bis(halotriazolium-pyrene) hosts for the sensing of pyrophosphate and dihydrogenphosphate in acetone via pyrene excimer formation (akin to the above mentioned 4.XB system). 97 Fluorescent H 2 PO 4 À sensing in ACN was also reported by emission enhancements of a BINOL-bis(triazolium) system. 98 Aggregation-induced emission (AIE) has over the last two decades rapidly emerged as a new paradigm in a plethora of luminescence applications, but remains notably underexplored in the context of host-guest ion sensing. 99 Recently, Docker and Shang et al. conducted a systematic binding and sensing study of a range of iodo-triazole appended tetraphenylethene (TPE) receptors as AIE platforms. 100 The tetra-XB receptor 5.XB displayed expectedly strong Cl À recognition in d 8 -THF and responded to various anions via signicant uorescent enhancements, with a notable halide selectivity, as shown in Fig. 10. The origins of this response were ascribed to anion binding-induced AIE, as supported by DLS and TEM  measurements, conrming the presence of z100 nm sized particles only in the presence of anions. This uorescent AIE response was notably not observed for the HB analogue 5.HB nor for a weaker XB donor receptor analogue containing phenyl instead of peruorophenyl substituents. Similarly, the mono-XB TPE derivative displayed no signicant emission response. In spite of strong halide binding by all three possible doublysubstituted XB TPE isomers, only the 1,1-diXB-ethene isomer exhibited chloride-induced AIE, highlighting the importance of the spatial orientation of XB donor sites to effect AIE.
The authors further demonstrated that the E and Zderivatives of the other doubly-substituted 1,2-diXB-ethene XB TPE receptor can be interconverted by light, whereby the relative composition in the photostationary state is dependent on anion presence.
Another recent research focus in the development of XB uorescent sensors has been their operation in (pure) aqueous environments. To this end Beer and co-workers have developed a range of water-soluble optically responsive XB receptors, including a benzene bis(iodotriazolium) host for emission turnon sensing of ReO 4 À in HEPES buffer (pH ¼ 7.4) 32 as well as a naphthalimide-appended XB foldamer receptor for sensing of I À in water via uorescence enhancement. 31 In another example, the Beer group recently developed the XB coumarin-appended receptor 6.XB as a hydrosulde (HS À ) selective uorescent probe. 101 Formed upon dissolution of the toxic H 2 S gas, the sensing of HS À remains a formidable challenge, in spite of its increasing relevance in environmental and medicinal settings. 102,103 Thus far, the vast majority of HS À sensors are irreversible optical chemodosimeters, 104,105 while host-guest recognition of this anion remains largely underexplored. 106-108 Receptor 6.XB not only presents a rare example of a reversible supramolecular HS À host but is capable of selective sensing of this analyte in water. 101 As shown in Fig. 11, addition of up to 10 equiv. of HS À to a buffered solution of 6.XB induced notable coumarin emission quenching of up to 60%, while neither Cl À , Br À nor I À induced an appreciable response. While the XB sensor displayed strong HS À binding (K ¼ 16 500 M À1 ) and a sensitive uorescence response (LOD ¼ 14 mM), the HB analogue 6.HB did not display any anion detection capability. These ndings were further corroborated by DFT and molecular dynamics simulations, highlighting a unique potency of XB for the recognition of HS À .  In addition to the organic uorophores discussed above, various organometallic and transition metal-based emissive XB sensors have been developed. Their typically modular synthesis and bright, highly tuneable emission proles renders them potent motifs in optical ion sensors. 109 This has been exploited in a range of XB uorescent anion sensors, an early example of which is the neutral, bimetallic pyrimidine-(iodo)triazole 7.XB/ HB system containing Re(I) carbonyl reporter groups (Fig. 12). 110 Preorganisation and polarization of the triazole binding site by this organometallic motif enabled strong binding of a range of halides and oxoanions in 1 : 1 CDCl 3 /MeOD, with strongest binding observed for iodide at 7.XB with K > 10 000 M À1 . Preliminary sensing studies in ACN revealed absorbance changes and luminescence enhancements of both receptors in the presence of the halides, phosphate and sulfate.
A similar design concept was exploited in receptors 8a-b.XB/ HB by the group of Ghosh. 111,112 In ACN 8a.XB displayed signicant emission enhancement of the Ru(phen) 2 py-triazole MLCT band of 5 and 17-fold in the presence of HP 2 O 7 3À and H 2 PO 4 À , respectively, while a large range of other oxoanions and halides did not induce signicant responses. 111 This correlated with stronger binding of the latter anion, with a 1 : 1 host-guest stoichiometric binding constant of K ¼ 194 000 M À1 , bound 3.5-fold more strongly than pyrophosphate. This was also conrmed by competition experiments; even in the presence of 10 equiv. of various competing anions the sensor response towards 1 equiv. of H 2 PO 4 À was largely unaltered. In addition, the authors reported a notable increase in the MLCT luminescence lifetimes of the probe, from z6 ns of the free receptor to z34 and 109 ns in the presence of HP 2 O 7 3À and H 2 PO 4 À , respectively. For the HB analogue 8a.HB the sensing performance towards these anions was expectedly reduced, with lower binding constants, larger LODs as well as reduced lifetime enhancements. This is also reected in a signicant sensing performance for 8a.XB in up to 20% water in ACN (albeit with lower response magnitudes), while 8a.HB was incapable of sensing the phosphate anions in mixtures containing 10% or more water.
In a subsequent study, the same group also investigated the more polarized pentauorophenyl appended receptor analogue 8b.XB. 112 Unsurprisingly, the XB receptor displayed enhanced binding of both HP 2 O 7 3À and H 2 PO 4 À (K ¼ 8.9 Â 10 5 and 2.76 Â 10 6 M À1 in ACN, respectively), over 10-fold larger than the benzylappended 8a.XB. In addition, 8b.XB also displayed a larger switch-on response of 25-fold in the presence of H 2 PO 4 À , while the enhancements in the presence of HP 2 O 7 3À were comparably somewhat attenuated (3.6-fold increase). The LOD was low towards both anions (z11 and 91 nM, respectively).
The rst example of a XB receptor containing the ubiquitous, luminescent cyclometallated Ir(ppy) 2 -motif 113,114 was reported by Schubert and co-workers in 2020. 115 Containing an additional 4,4-bis-iodotriazole bipy ligand, 9.XB displayed signicantly enhanced Cl À binding (60 000 M À1 ) in comparison to its HB congener in ACN (5000 M À1 ). Both Br À and OAc À were also bound, albeit weaker. 9.XB exhibited a signicant luminescence response towards chloride with a low LOD of 11 nM, while the perturbations induced by the other anions were notably smaller.
Acyclic uorescent XB sensors based on other XB donor motifs include, for example, iodo-naphthoquinone receptors for sensing of SO 4 2À in ACN 116 and iodo-pyridinium receptors for sensing of various halides and oxoanions in DCM. 117 3.2.2 Interlocked luminescent XB sensors. As a result of their well-dened three-dimensional binding cavities, mechanically interlocked receptors have garnered signicant attention in ion recognition and sensing. 40,41,50,118,119 Both rotaxane and catenane hosts advantageously display enhanced binding strength and selectivities in comparison to acyclic or macrocyclic systems; mechanical bond effect properties that synergise particularly well with sigma-hole donors, as increasingly exploited in anion supramolecular chemistry. 30,120,121 Unsurprisingly, signicant attention has been directed at the incorporation of various reporter groups into these interlocked ion receptors, in particular luminescent reporters.
The rst example of a uorescent XB interlocked host, and, to the best of our knowledge, the rst example of XB-mediated uorescent sensing in general, was reported by Caballero et al. in early 2012. 122 In ACN the bis-bromo-imidazolium [2]catenane 10.XB, containing naphthalene reporter groups, displayed selective uorescence switch-on only in the presence of Cl À or Br À , with strong binding of 3.71 Â 10 6 M À1 and 148 000 M À1 , respectively (Fig. 13). In contrast, a large range of other anions, including F À , I À , AcO À , H 2 PO 4 À , NO 3 À and HCO 3 À did not induce any uorescence response. Similarly, the monomeric macrocyclic host precursor did not respond to any anions, highlighting the unique selectivity imbued by the preorganized, interlocked catenane XB binding cavity. Another structurally related hetero-[2]catenane containing one XB iodo-triazolium macrocycle as well as a HB isophthalamide macrocycle component, was reported for uorescent sensing of halides and oxoanions in ACN. 123 All tested anions induced emission enhancements of the naphthalene emission, which were largest for the oxoanions AcO À and H 2 PO 4 À (+73 and +58% intensity increase in the presence of 20 equiv. anion). The response towards the halides was notably smaller with +29, +13 and +4% emission modulation for Cl À , Br À and I À , respectively, thereby displaying a contrasting oxoanion selectivity in comparison to the bromo-imidazolium [2]catenane sensor 10.XB. Fluorescent reporter motifs have also been incorporated into various rotaxane receptors. This includes, for example, a XB tris(iodo-triazole) rotaxane containing an anthracene reporter appended to the rotaxanes' macrocycle, capable of Cl À sensing in ACN. 124 Lim et al. also prepared a chiral XB [3]rotaxane uorescent sensor 11.XB for biologically relevant dicarboxylates (Fig. 14). 125 1 H NMR titrations in CDCl 3 /CD 3 OD/D 2 O 60 : 39 : 1 revealed signicantly different binding modes between the rotaxane host and chloride and the dicarboxylates S-glutamate, R-glutamate, fumarate and maleate. While Cl À was bound in a 1 : 2 hostguest stoichiometry with the halide binding within each individual interlocked cavity, dicarboxylate binding proceeded in a 1 : 1 fashion via formation of sandwich-type complexes.
From these 1 H NMR studies signicant Cl À binding (K 1:1 ¼ 2610 M À1 ) was ascertained, while the chemical shi perturbations in the presence of the dicarboxylate guests were too small to be reliably analysed. However, uorescence sensing studies in the same solvent system revealed strong dicarboxylate recognition with K of up to 35 200 M À1 for S-glutamate, as re-ected in almost complete BINOL uorescence quenching. Impressively, binding of the R-glutamate enantiomer was 5.7fold attenuated, attesting to the unique potential of chiral interlocked hosts and sensors; the axle alone not only bound both guests much more weakly (K z 1600 M À1 ) but also displayed no signicant degree of enantiodiscrimination.
A range of XB strapped-porphyrin receptors including BODIPY-containing rotaxanes 12a-b.XB were recently reported by Tse et al. (Fig. 15). 79 The XB strapped-porphyrin macrocycle alone displayed signicant changes (red-shi) in its Soret absorption band upon titration with halides in acetone, revealing strong anion binding which was enhanced up to 10 000-fold in comparison to the unfunctionalized parent Zntetraphenylporphyrin. This XB macrocyclic component was then integrated into a range of [2]rotaxanes, which showed signicantly enhanced halide binding affinities in comparison to an analogous porphyrin-free rotaxane in d 6 -acetone and d 6acetone/D 2 O 98 : 2 as elucidated by 1 H NMR studies. This can be attributed to an enhanced preorganisation and polarization of the interlocked binding cavity by axle-triazole Zn-porphyrin coordination. Unfortunately, this negated the ability of the metallo-porphyrin to act as a chromophoric reporting group; even in the presence of a >1000-fold excess of halides no colorimetric changes were observed. In order to restore the sensing capabilities of the rotaxanes, uorescent BODIPY reporter groups were incorporated as axle components into 12a-b.XB. In acetone, both rotaxanes responded to Cl À , Br À , I À , OAc À and SO 4 2À via BODIPY uorescence quenching, whereby binding strength (and response magnitude) were larger for the more polarized peruorophenyl-containing 12b.XB for all anions, as representatively shown for Cl À in Fig. 15. In the presence of 2% water in acetone, only 12b.XB responded to Cl À and Br À with K ¼ 1090 and 650 M À1 , respectively, while 12a.XB did not respond to any anion. By virtue of the redox-activity of the Zn-porphyrin motif, the rotaxanes were also investigated as voltammetric anion sensors. In DCM, all rotaxanes displayed large cathodic voltammetric perturbations in the presence of HSO 4 À , OAc À and Cl À , of up to À222 and À252 mV for OAc À and Cl À with 12a.XB, respectively. These XB rotaxanes represent rare examples of dual optical and electrochemical sigma-holemediated sensing.
The incorporation of transition metal-based luminescent reporters into interlocked XB hosts has also been investigated by Beer and co-workers, including the all-halogen bonding rotaxane 13.XB (Fig. 16). 126 In ACN containing 10 or 20% H 2 O this Re(I)(bipy)-containing receptor displayed emission quenching in the presence of Cl À , Br À and I À while various oxoanions only induced minor or negligible perturbations. Halide recognition proceeded with 1 : 2 host-guest  stoichiometry in both solvents with K 1:1 of up to 138 000 M À1 for iodide, while Cl À and Br À were bound weaker and F À did not bind at all. Even in ACN/H 2 O 1 : 1 selective halide uorescence quenching was still observed, following the same binding trend (I À > Br À > Cl À ) with K 1:1 of up to 24 000 M À1 .
A similar Re(I)(CO) 3 Cl-bistriazole rotaxane was also developed, which was however not luminescent. 127 Langton et al. also integrated a Ru(bpy) 3 2+ reporter motif into a water-soluble XB rotaxane and demonstrated Br À , I À and SO 4 2À sensing in pure water, albeit with modest emission enhancements in the presence of excess anion of 3, 6 and 20%, respectively. 128 In spite of a larger maximum response for sulfate, the halide anions were bound more strongly as their maximum emission response was reached at a concentration of 1 mM, while a higher concentration of 8 mM SO 4 2À was required to induce signal saturation.

Chalcogen bonding luminescent sensors.
In comparison to the numerous examples of luminescent XB sensors, the exploitation of ChB in optical sensing remains very rare. To the best of our knowledge, the rst examples of emissive ChB sensors 14a-c.ChB (Fig. 17) were reported by the group of Matile in 2016. 36 These dithienothiophenes (DTTs) have emerged as powerful (supra)molecular scaffolds with numerous applications, in particular as uorescent probes. 129 Particularly notable in the context of this review is their surprisingly potent ChB donor capability arising from convergently arranged sulfurdonor atom based s-holes, polarized through the sulfone backbone.
This has been exploited in catalysis, 1 anion binding and, in the afore-mentioned seminal work, for anion transport. 36 Developed as transmembrane anionophores, DTT receptors 14a-c.ChB were also investigated as anion receptors and optical sensors. All three receptors displayed signicant uorescence emission quenching upon addition of up to 20 mM chloride in THF, with largest quenching and strongest binding of 885 M À1 and 204 M À1 observed for 14a.ChB and 14c.ChB, respectively. Both 14b.ChB (149 M À1 ) and a mono-cyano derivative of 14a.ChB (69 M À1 ) displayed weaker binding as well as smaller uorescent responses. All three receptors also underwent small changes in absorbance in the presence of chloride. In contrast, as expected PF 6 À did not induce any optical responses, while smaller uorescence emission quenching of 14a.ChB and 14c.ChB was also reported in the presence of NO 3 À , in good agreement with weaker binding of this anion of 161 M À1 to 14a.ChB.
Beer and co-workers reported a series of chiral ChB/XB/HB (S)-BINOL based triazolium receptors 15.ChB/XB/HB for the recognition and uorescent detection of stereo-and geometric dicarboxylate isomers in acetone/H 2 O 85 : 15. 130 While 15.XB displayed signicant chiral discrimination in the recognition of the enantiomers of tartrate and N-Boc-glutamate, both 15.HB and in particular 15.ChB did not display signicant levels of enantioselectivity towards these chiral anion guests as elucidated by 1 H NMR binding studies. In contrast, all receptors displayed a considerable degree of binding discrimination of the geometric isomers maleate/fumarate as well as phthalate/ isophthalate with a preference for the more extended fumarate or isophthalate in all cases. For the former pair, 15.ChB displayed the largest discrimination with K fum /K mal ¼ 5.5, signicantly better than 15.HB with K fum /K mal ¼ 2.0, while 15.XB decomposed upon exposure to malate.
Similarly, both sigma-hole hosts displayed enhanced preference for isophthalate over phthalate in comparison to the HB congener. Interestingly, all three hosts displayed signicantly different uorescent anion sensing properties (Fig. 18).
While addition of all anions induced large emission enhancement for 15.XB, 15.HB showed quenching in all cases. In contrast, 15.ChB showed more nuanced response patterns with emission enhancements towards (iso)phthalate and quenching in the presence of fumarate/maleate. In addition, the ChB host also displayed unique changes in the emission  wavelengths with opposite, hypso-and bathochromic shis for isophthalate and phthalate, respectively.
Recently, Che and co-workers reported a uorescent ChB sensor for the detection of the toxic trimethylarsine gas. 131 This was achieved by formation of bundled nanobers constructed from the tris(benzoselenadiazole) receptor 16.ChB which responded to exposure of the analyte vapours by a decrease in emission intensity (Fig. 19). With a LOD of 0.44 ppb and a fast response time of z3 s, this sensor performed signicantly better than its sulfur-donor benzothiadiazole analogue (LOD ¼ 67 ppb, response time of 54 s). This is indicative of the response arising from ChB-mediation recognition, as further supported by DFT calculations. The sensor displayed an impressive level of selectivity, with no notable response to a large range of solvent vapours, including H 2 O, methanol, ethanol and acetone at signicantly higher levels. In addition, selective trimethylarsine detection was also demonstrated in complex matrices, including car exhaust, smoke and hair spray.
The same group also developed a structurally related ChB benzoselenadiazole sensor for assessment of meat freshness by uorescent sensing of gaseous dimethyl sulde. 132

Electrochemical sensors
Owing to their low cost, high sensitivity, exibility, and scalability, electrochemical sensors are at the forefront of sensor development, 133 in particular for the sensing of biomolecules, 134 but also for small molecules and ions. 40 The latter most notably includes ion-selective electrodes, which, as alluded to in Section 2.1, are thus far the only widely and generically applied ion sensors. This can in part be attributed to a century-long development of potentiometric techniques, their low cost and (operational) simplicity. [46][47][48][49] Nevertheless, these are not available for various (an)ions and, depending on the specic application scenario, can fail to address certain sensing criteria (e.g. selectivity, or the capability to monitor small changes in concentration), see also Section 4.2. § This has sparked signicant research into alternative electroanalytical supramolecular host-guest ion sensing methodologies, in particular voltammetric sensors based on redox-active receptors as discussed in the following Section.

Redox-active sensors
4.1.1 Solution-phase voltammetric anion sensing. The integration of redox-active reporter groups, in particular ferrocene (Fc), into synthetic receptors is a well-established approach to generate potent sensors whose voltammetric properties are dependent on the presence of the bound guest species. 40,71 Specically, recognition of a Lewis basic (typically anionic) guest enhances the electron density at the redox active receptor, stabilising the higher oxidation state, which is reected in a cathodic voltammetric perturbation (shi to more negative potentials, easier oxidation) of the redox couple. This change in the receptor's half-wave potential (E 1/2 ) is then readily measurable by simple voltammetric techniques such as cyclic voltammetry (CV), differential pulse voltammetry (DVP) or square-wave voltammetry (SWV), see Fig. 20.
In turn, voltammetric modulation of the receptor's redox state affects the guest binding properties, with stronger anion binding in the higher, more cationic oxidation state (K Ox ) than  in the lower, neutral or anionic redox state (K Red ). The specic signalling pathways and fundamentals that underpin these observations are well-established in the literature, but will not be discussed in detail herein. 40,[135][136][137][138] However, note that in the most general case the magnitude of the voltammetric perturbation is given by and is thus only dependent on the ratio of K Ox /K Red (oen denoted the binding enhancement factor, BEF). 40 This is to say that the signal magnitude in a voltammetric (an)ion sensor is determined by how strongly guest binding is affected by receptor oxidation/ reduction. From this consideration it becomes apparent that a stronger electronic communication between the redox and receptive sites is a key factor for sensor performance. It is thus not surprising that sigma hole interactions, which, as discussed in Section 2.2, display a high degree of electronic tuneability, are particularly potent in voltammetric (anion) sensors. 139 The rst demonstration of a high sensitivity of redox control of XB was reported by Schöllhorn and co-workers in 2014, wherein it was shown that the voltammetric properties of redox active Lewis bases can be modulated in the presence of neutral XB donors (note that this is the "reverse" case as depicted in Fig. 20 and in subsequent examples). 140 For instance, in ACN the reduction of tetrachloro-p-quinone (TCQ) is facilitated in the presence of iodo-peruoro-alkynes/arynes. Specically, CV experiments showed that single-electron reduction of TCQ to TCQ À c is not perturbed by addition of 1-iodo-peruorohexane, while the second reductive couple TCQ À c/TCQ 2À undergoes signicant anodic voltammetric shis of up to 140 mV in the presence of up to 100 equivalents of XB donor. This is indicative of XB formation, and concomitant decrease in electron-density, which occurs only for the stronger, dianionic Lewis base TCQ 2À .
Shortly thereaer the Beer group reported the rst examples of redox active XB iodotriazole voltammetric anion sensors 17.XB and 18.XB (Fig. 21A). 141 In this case, and all following examples, the XB receptor itself is redox-active, such that the XB donor strength is electrochemically modulated and sensing of redox-inactive Lewis basic analytes, in particular anions, is enabled. In ACN both receptors responded to presence of up to 10 equiv. of Cl À or Br À with moderate cathodic shis of zÀ30 and zÀ20 mV, respectively (Fig. 21B), which is notably larger than the response of their HB congeners 17.HB and 18.HB. In particular the response of 18.HB was strongly diminished with À6 and 0 mV for Cl À and Br À , respectively, highlighting the crucial role of the XB interaction in sensing the halide anions.
Subsequently, a range of XB Fc-containing acyclic receptors were prepared by different groups. For example, Zapata, Caballero and Molina reported trisferrocene-bis((iodo)triazole) receptors 19.XB/HB (Fig. 22) as oxoanion sensors in DCM/ACN 1 : 1. 142 Notably, the receptors display two redox waves as a result of strong electronic coupling between the two chemically inequivalent Fc environments, whereby the outer Fc motifs are simultaneously oxidised rst, while the inner Fc is subsequently oxidised at a z400 mV higher potential. In addition, the sensors display comparably complex voltammetric response patterns, characterised by a two-wave slow exchange behaviour and response magnitudes that differ signicantly for both redox waves. For example, addition of OAc À or SO 4 2À to 19.XB did not perturb the rst, more cathodic redox wave, but induced     143 Receptor 20.XB notably displayed larger responses towards this anion of À40 mV over Cl À , Br À or OAc À (max. À22 mV for Br À , all at 10 equiv.) as well as signicantly enhanced signal magnitudes in comparison to 20.HB (À18 mV for N 3 À ).
The chiral 21.XB represents a rare example of voltammetric enantioselective sensing of various chiral anions, achieved in ACN. 144 This (S)-BINOL-based XB probe displayed a larger response for the R-enantiomer of both N-Boc-alanine and N-Boc-leucine with DE R /DE S of 1.11 and 1.41, respectively and a preferential response towards the S-enantiomer of BINOLphosphate with DE R /DE S ¼ 0.67. These voltammetric enantioselectivities are not only in very good agreement with those obtained by 1 H NMR binding titrations of the neutral receptor but are also larger than those observed in a previous Fcurea HB sensor. 145 These observations highlight the potential of XB systems not only as potent voltammetric sensors with typically enhanced response magnitudes in comparison to HB analogues, but also enhanced enantiodiscrimination, presumably arising from the stricter geometric preferences imposed by XB.
In an effort to further enhance the selectivity of such voltammetric anion sensors, Lim and Beer recently integrated a Fcreporter group into an all-XB rotaxane 22.XB. 146 In the competitive solvent mixture of ACN/acetone/H 2 O 45 : 45 : 1, this sensor displayed a modest but notably selective response towards excess Br À of À22 mV over Cl À and SCN À . This selectivity is in good agreement with the binding preference of the native rotaxane as elucidated by 1 H NMR binding titrations (Fig. 23).
The groups of Beer as well as Schöllhorn and Fave also investigated a range of other redox transducers in XB anion sensors, such as viologen-based systems for detection of various halides, whereby all studies revealed an important contribution of XB in obtaining (enhanced) voltammetric responses. [147][148][149] The latter groups further conducted a range of systematic studies into XB iodo-tetrathiafulvalene (TTF) voltammetric sensors. 138,150 As shown in Fig. 24, iodo-TTF 23.XB displays two reversible oxidative couples in DMF, corresponding to step-wise one-electron oxidation to 23.XB + c and 23.XB 2+ , which both respond to the presence of increasing chloride concentrations by well-dened, continuous cathodic shis. As expected, various control experiments proved that XB formation was the crucial driving force in halide recognition and sensing. 138,150   The sensor further displayed somewhat smaller cathodic perturbations in the presence of Br À , while OTf À , NO 3 À and H 2 O did not induce any response (Fig. 25). By tting of the voltammetric binding isotherms to a 1 : 1 host-guest stoichiometric Nernst binding model the authors further extracted absolute halide binding constants to all receptor oxidation states. 40,138 Unsurprisingly, the neutral, native 23XB displayed only very weak halide anion binding constants (#20 M À1 ), while binding to the monocationic 23XB + c was signicantly switched on with K ¼ 425 and 131 M À1 for Cl À and Br À , respectively. A further increase in chloride binding to 23XB 2+ of K ¼ 6730 M À1 was extracted, while the analogous binding constant for Br À could not be obtained as Br À oxidation overlapped with the second oxidative TTF couple in CV. These observations saliently highlight the unique advantages of voltammetric anion sensors; the transient generation of a more cationic, higher oxidation state increases anion binding to such an extent that sensing in competitive solvent media, in which the native receptor oen displays negligible anion binding, is possible. This concept was also recently exploited for the sensing of anions in competitive aqueous/organic solvent systems at a range of interfacial XB anion sensors, as discussed in more detail in Section 4.1.2. 57,71,72,137 The same groups later reported a systematic investigation of rarely studied electrolyte effects on the anion sensing performance of a methylated iodo-TTF derivative of 23.XB. 150 The authors showed that different electrolyte anions BF 4 À , MsO À , TfO À , NO 3 À , ClO 4 À , PF 6 À or BAr F 4 À ([tetrakis[3,5bis(triuoromethyl)phenyl]borate]) can signicantly inuence the sensing properties of (XB) voltammetric anion sensors. For instance, the cathodic shi perturbation of XB sensor trimethyliodo-TTF towards 100 equiv. Cl À ranged between À36 and À49 mV, corresponding to an up to 2.6-fold difference in K Ox , depending on the electrolyte. These observations highlight that even "non-coordinating" electrolyte anions may signicantly compete with anion binding and associated signal transduction in redox-active sensors. This was recently corroborated by a systematic comparison of NMR, UV-vis and voltammetrically determined anion binding constants in XB viologen derivatives in the absence and presence of electrolytes. 149 In 2022, Hein et al. reported the rst examples of ChB voltammetric anion sensors including the dicationic telluroviologen derivative 24.ChB as well as the neutral pyridine bis(ferrocenyltellurotriazole) 25.ChB (Fig. 26). 62 The telluro-viologen 24.ChB displayed moderately strong halide binding in competitive CD 3 CN/D 2 O 9 : 1 with a modest preference for bromide (K ¼ 1036 M À1 ) while its lighter Se congener bound all halides much more weakly (K ¼ 182 M À1 for Br À ), yet still stronger than the HB viologen analogue (K ¼ 139 M À1 ), conrming a signicant ChB participation in anion recognition. Voltammetric anion sensing studies in the same solvent system conrmed signicant cathodic responses of the rst reductive viologen couple of the telluoroviologen 24.ChB, which were again largest for bromide (DE max ¼ À61 mV), and slightly smaller for chloride (À57 mV) and iodide (À49 mV). In contrast, the oxoanions HSO 4 À and NO 3 À induced smaller responses of #À36 mV. These perturbations were only observed for the rst reductive couple; the second reduction couple was not perturbed in the presence of any anion, see Fig. 27. This indicates that upon rst mono-electron reduction, the receptor's potent ChB ability is switched-OFF i.e. the bound anion is expelled. A further reduction has thus no additional effect on anion binding and no shis of the second couple are observed. Importantly, both the lighter ChB seleno-congener as well as the unfunctionalized HB viologen responded to anions much more weakly, with largest responses towards Cl À of À22 and À17 mV, respectively. Of further note is that 24.ChB also responded to the halides via naked eye-visible changes in absorbance (red-shi), thereby acting as a dual-output anion sensor. In contrast to the reductive switch-OFF telluro-viologen system, the neutral ferrocenyltellurotriazole receptor 25.ChB  was investigated as a ChB switch-ON sensor via ferrocene oxidation.
In  57 This conrms a particularly potent redox-dependent bindingmodulation of ChB (large BEF and large DE), enabled by a high sensitivity of ChB on its electronic environment 28 as well as the uniquely close spatial coupling of the redox and Te-donor binding sites, a design principle with signicant future potential. Importantly, these ndings establish redox-control of ChB as a powerful, reversible approach for high delity switch-OFF or switch-ON modulation of ChB anion recognition and sensing.
4.1.2 Interfacial redox-active anion sensors. In another recent development, redox-active XB receptors were immobilised onto electrode-surfaces to furnish surface-conned anion sensors. This is associated with numerous advantages over solution-phase sensing, most importantly enhanced sensory responses, circumventing solubility constraints, facile device integration, potential for sensor reuse and sensing under ow. 40,151 The rst example of such an interfacial XB voltammetric sensor, the self-assembled monolayer (SAM) of a bisiodo-TTF derivative 26.XB SAM (Fig. 28), was reported in 2019 by Fave, Schöllhorn and co-workers. 152 In ACN, this interface voltammetrically responded to the halides Cl À and Br À , with cathodic perturbation of the rst TTF oxidative redox couple of z À150 mV towards Cl À . Of particular note is that this behaviour differs distinctly from that of the same receptor studied in solution. Under diffusive conditions, the halide response is not only smaller (up to z À95 mV in presence of 200 equiv. Cl À ),   is also characterised by continuous cathodic shis (as also observed for the related 23.XB, see Fig. 24 and 25), however these studies were, for solubility reasons, carried out in a different solvent system of ACN/DMF 3 : 7. In contrast, the response of 26.XB SAM follows a more complex slow-exchange two-wave pattern with emergence of a new peak at lower potentials, a result of altered kinetic binding proles. These results nicely illustrate the afore-mentioned advantages of interfacial sensing, that is circumvented solubility constraints and improved response magnitudes. The latter was justied by elucidation of the Cl À binding constants to 26.XB SAM and 26.XB SAM + c, which were with K Red z 1000 M À1 and K Ox z 570.000 M À1 , not only individually larger than those in solution, but whose ratio (i.e. the BEF) was also signicantly enhanced, corresponding to a larger response magnitude (DE f K Ox /K Red ).
An enhanced interfacial response of the bis(ferrocene-(iodo)triazole) sensors 27.XB/HB SAM towards the oxoanions HSO 4 À , H 2 PO 4 À and NO 3 À in various ACN/H 2 O mixtures of up to 30% water was also reported by Patrick et al. in 2021. 57 Interestingly, the HB congener 27.HB SAM displayed a slightly enhanced response towards these oxoanions in comparison to 27.XB SAM , while under diffusive conditions 27.XB dif outperformed 27.HB dif in response to all anions, including Cl À and Br À , in a range of ACN/H 2 O mixtures of up to 20% water. This unexpected observation potentially arises from differing interfacial receptor organizational or hydration differences as elucidated by various surface analyses. Particularly noteworthy in this study is a rare demonstration of XB/HB (interfacial) voltammetric sensing in highly competitive aqueous media of up to 30% water (in which the diffusive receptors are not soluble), again highlighting the utility of surfaceimmobilisation in generating potent, potentially real-life relevant electrochemical anion sensors. This work also presents the rst comprehensive study into solvent effects in voltammetric anion sensors. Unsurprisingly, the voltammetric shi magnitude of both sensors, in solution and at the surface, generally decreased upon increasing water content, particularly strongly for H 2 PO 4 À , a reection of its large hydration enthalpy. A noteworthy exception to this trend is the solution-phase performance of 27.XB dif , whose response to the halides was largely independent of water content (note that this effect could not be studied at the interfacial receptors due to poor voltammetric reversibility of 27.XB/HB SAM in the presence of halides). Importantly, this trend was not observed for 27.HB dif ; with increasing water content the anion sensing performance of 27.XB dif relatively increased in comparison to the HB sensor, attesting to the potency of XB anion sensing in aqueous media. A detailed investigation into the transduction mechanisms that govern the response mechanisms and enhanced signal magnitudes of interfacial voltammetric sensors was recently reported by Hein et al. 137 As shown in Fig. 29, the XB/HB ferrocene-isophthalamide-(iodo)triazole interface 28.XB/HB SAM displayed signicantly enhanced sensory responses towards a range of oxoanions as well as halides in ACN/H 2 O 99 : 1.
In analogy to the 27.XB/HB sensor system, the interfacial response towards the oxoanions HSO 4 À , H 2 PO 4 À and NO 3 À was slightly augmented for 28.HB SAM , while 28.XB SAM displayed a modest preference towards the halides Cl À and Br À , in agreement with previous observations of a typical XB recognition preference towards (soer) halides. 14,23 In good agreement with the above-described voltammetric studies is also the consistently augmented solution-phase performance of 28.XB dif towards all anions. All of these observations were rationalised in the context of a novel dielectric model, highlighting the importance of through-space and through-bond charge interactions and their screening in environments of different dielectric.
As a result of its well-dened anion sensing performance and high voltammetric stability the groups of Beer and Davis further developed 28.XB/HB SAM as anion sensors for real-time continuous ow sensing. The detection of anions in aqueous media under ow conditions is of high relevance across various applications, including long-term health and water monitoring, but remains underdeveloped. Interfacial supramolecular sensors are ideally suited to address this challenge as they can be easily re-used by simple washing. To demonstrate this capability, Patrick and Hein et al. developed a 3D-printed electrochemical ow cell (Fig. 30A) through which electrolyte was continuously pumped over a 28.XB/HB SAM -modied gold electrode. 72 A continuous sensor signal readout (that is the E 1/2 ) of the sensor was obtained by repeat SWV voltammetry and analysis of the voltammograms with a custom MATLAB script, affording a highly stable signal baseline with a temporal resolution of z4 s. Injection of anion aliquots (HSO 4 À , H 2 PO 4 À or Cl À ) into the ow then induced response spikes in the sensograms (Fig. 30B), the magnitudes of which were pleasingly identical to that obtained under standard, "static" conditions. Importantly, upon washing with fresh electrolyte, the sensor's response quickly returned to its baseline, conrming complete anion removal. Impressively, the sensor could be continuously operated over a 4.5 h period (corresponding to 3700 voltammetric scans), with a highly reproducible response to repeat HSO 4 À injections and minimal baseline dri of #5 mV (Fig. 30C). Surface-immobilisation of ion receptors is associated with a further unique advantage over solution-phase sensing; an ability to utilise other electroanalytical techniques. This most notably includes electrochemical impedance/capacitance spectroscopy (EIS/ECS). The former is well-established for the sensing of ions at redox-inactive receptive electrode surfaces, whereby a signal-generating solution-phase redox probe (typically ferri/ferrocyanide) is employed. [153][154][155] Upon interfacial ion recognition, electrostatic interactions between the charged redox probe and the receptive surface are altered such that a change in the charge-transfer resistance is observed (Faradaic impedance).
In contrast, faradaic capacitance spectroscopy relies on changes in the capacitive, that is charge-storing, properties of a surface-bound redox transducer. 156 This redox capacitance C r is highly sensitive to changes in local (dielectric) properties and is well-established for biosensing. [157][158][159] In 2021, Patrick and Hein et al. demonstrated for the rst time the utility of this approach for ion sensing. 71 This study was carried out on the same XB receptive interface 28.XB SAM , enabling a direct comparison with the afore-discussed voltammetric sensing format. Redox capacitance spectroscopy at a xed AC frequency was employed to resolve the interfacial redox capacitance C r which reports on the redox density of states (DOS) of the electro-active interface. Resolving this DOS (green triangles, Fig. 31A) affords, in the rst instance, analogous information as standard SWV (black trace), i.e. it reports on anion binding-induced cathodic shis (Fig. 31B).
However, in contrast to voltammetry, each point within this C r DOS distribution is recorded at equilibrium, i.e. does not require a potential sweeping. Instead, C r can be continually measured at a constant, freely chosen electrode potential and provides a simple, constant, direct sensor readout. For example, if C r is continually monitored at the initial E 1/2 of the interface then anion binding induces a drop in signal, as C r now lies in the anodic tail of the redox distribution. The sensing isotherm obtained in this manner is similar to that obtained by standard voltammetry (Fig. 31C), however the binding and response are somewhat enhanced due to a self-amplication effect in the redox capacitive format.  As a result of its direct readout, necessitating no further data analysis, and its high temporal resolution (z2.5 s) this novel methodology is ideally suited for real-time, continuous ow ion sensing, as investigated in the same 3D-printed ow cell as used for previous voltammetric studies. As shown in Fig. 32A, injection of HSO 4 À aliquots of increasing concentration induced noticeable response spikes, whose magnitude and "direction" are dependent on the applied electrode potential. In addition to this signal switch-on/off control the apparent binding constant (i.e. "steepness") of the response isotherm can be modulated by judicious choice of potential (Fig. 32B) by up to 1 order of magnitude (K app at +100 mV ¼ 320 M À1 vs. 36 M À1 at À200 mV). Concomitantly, the sensor's LOD can be tuned in this manner, and is with z45 mM (for measurements at E 1/2 ), lower than in an optimised voltammetric format, 72 which cannot be additionally tuned. With a higher temporal resolution, direct sensor readout as well as improved and tuneable analytical performance, this redox capacitive sensing format is thus vastly superior to standard voltammetry. In addition to supporting improved ion sensing capabilities, preliminary investigations suggest that the redox capacitive readout maybe used to elucidate interfacial host-guest binding kinetics and thus also presents a novel tool in the fundamental study of interfacial electro-active supramolecular host-guest systems.

Other electrochemical sensors
4.2.1 Capacitive sensors. Electrochemical capacitance/ impedance spectroscopy can also be carried out in an entirely non-faradaic format, that is in the absence of either a solutionphase or surface-bound redox probe. In this case the interfacial non-faradaic capacitance of receptor-modied electrodes can serve as a transducer for an ion binding event, which is wellestablished for the sensing of cations at crown-ether modied electrodes. 160,161 Exploiting the uniquely potent performance of XB for anion sensing in water, Hein et al. recently demonstrated, for the rst time non-faradaic capacitive anion sensing at XB and HB foldamer molecular lms 29.XB/HB SAM (Fig. 33). 162 Fig. 32 (A) Redox capacitance response of 28.XB SAM towards HSO 4 À at E 1/2 (black), E À105 (green), E À200 (blue) and E +100 (red) under continuous electrolyte flow in a custom 3D-printed flow cell (see Fig. 31A). Each spike in (A) represents the response towards aliquots of HSO 4 À of increasing concentrations with absolute signal increasing or decreasing depending on the initial surface polarisation. (B) The corresponding baseline-corrected response isotherms. The dashed black line represents, for simpler comparison, the mirrored capacitance response at E 1/2 , highlighting the steeper response slope and enhanced anion binding magnitude at E 1/2 vs. at E À105 . Reproduced with permission from ref. 71 copyright 2021 American Chemical Society. In pure water this sensor responded selectively to the environmentally and biologically relevant charge-diffuse anions ReO 4 À , I À and SCN À by an increase in the interfacial capacitance ( Fig. 34A) while neither Cl À , Br À nor ClO 4 À induced any response. This is notably different to the recognition behaviour of the parent foldamer receptor in solution-phase, where 2 : 1 hostguest stoichiometric binding with ReO 4 À , I À , SCN À , Br À and ClO 4 À was ascertained via isothermal titration calorimetry (ITC), with b ¼ K 1 K 2 of up to 1.45 Â 10 10 M À2 for I À . In contrast, binding was signicantly attenuated at the surface for 29.XB/HB SAM (K # 360 M À1 ), and proceeds via formation of 1 : 1 host-guest complexes, as suggested by interfacial binding isotherm analysis according to the Langmuir adsorption model. As representatively shown in Fig. 34B, the XB sensor 29.XB SAM outperformed its HB congener in all cases, with not only higher maximum signal magnitudes but also increased binding strength.
The binding of ReO 4 À in particular was enhanced signicantly for 29.XB SAM (K ¼ 231 M À1 ) in comparison to 29.HB SAM (K ¼ 11 M À1 ). Relatedly, the LOD of 29.XB SAM was z3-fold improved in all cases and was lowest for I À (14 mM). The physicochemical origins of these capacitive sensor response patterns were also later analysed and justied within a mesoscopic model. 163 Of note is that, in principle, this sensor can, akin to the redox capacitive sensor 28.XB SAM , be operated at a xed frequency and freely chosen electrode potential and may thus be used for realtime ow anion sensing in pure water.
4.2.2 Potentiometric sensors. The most generically applicable commercial real-life relevant ion sensing methodology relies on the potentiometric determination of ions using ionselective electrodes (ISEs). 49,164,165 They respond to ingress of the analyte ion into an ion-selective membrane via a change in the electromotive force (i.e. a potential change) between the membrane-containing ISE and a reference electrode. In an ideal case the response of the ISE is, according to Nernstian principles, determined by E ¼ E 0 + S log a i , where E 0 is a constant, a i the activity of the ion and S ¼ 59 mV z i ; where z i is the charge of the ion. For a monovalent ion an ideal "Nernstian" response of 59 mV per decade change of ion concentration is thus expected. In order to render the ISE selective toward the target analyte, ion receptors (ionophores) are typically incorporated into the hydrophobic membrane component of the ISE. To date, the vast majority of anion ISEs rely on Lewis acid metal complexes or traditional HB receptors as ionophores. 40 Surprisingly, s-hole receptors have, despite their typically contrasting selectivity patterns and larger hydrophobicity, only very recently been explored in ISEs. Specically, the rst, and thus far only, example of a XB ionophore 30.XB was reported by Lim, Goh and co-workers for the potentiometric sensing of iodide in pure water. 166 Initial 1 H NMR studies in d 6 -acetone indicated convergent XB mediated I À recognition with moderate 1 : 1 host-guest stoichiometric binding of K ¼ 260 M À1 . In contrast, I À binding of the HB analogue 30.HB was signicantly attenuated with K ¼ 2.75 M À1 . Incorporation of the ionophores into polymeric membranes of varying composition produced a series of ISEs, all of which responded with a near-Nernstian response of z50 mV/decade and a LOD of z1.25 mM to iodide. The authors then conducted a range of selectivity studies in the presence of the potentially interfering anions Cl À , Br À , NO 3 À , SCN À and ClO 4 À . Based on the Hofmeister series, anions of low hydrophilicity, i.e. more hydrophobic ones, can more easily ingress into the membrane to induce the largest interference, as observed for a control membrane without ionophore, which displayed the expected selectivity pattern of: ClO 4 À > SCN À > I À > NO 3 À > Br À > Cl À . While incorporation of the 30.HB ionophore into the membrane did not appreciably alter this selectivity trend, the XB ionophore induced moderate enhancements in I À selectivity over all other tested anions with a notable, modest, preference for I À over SCN À , indicating that specic XB mediated I À recognition takes place within the membrane.
As shown in Fig. 35, the I À selectivity over the more hydrophilic halides was sufficiently large such that 10 mM Cl À or Br À did not interfere. In contrast, the more hydrophobic SCN À and in particular ClO 4 À signicantly interfered with I À determination at much lower levels of 0.1 mM. Nevertheless, this study provides an important rst foray into the exploitation of s-hole interactions in ISEs which will undoubtedly receive more

Other sensors
Akin to electrochemical sensors, chemiresistive sensors have emerged as simple, cheap and scalable sensing devices.{ They respond to analyte presence by changes in the conductance of a sensing material immobilised between two electrodes, a concept that has been exploited in particular for sensing of gases, 167 but also ions. [168][169][170] In 2016, the group of Swager demonstrated for the rst time the utility of XB "selectors" as host motifs to enable selective chemiresistive sensing of pyridine gas. 171 To this end they modied single-walled carbon nanotubes (SWCNTs) with haloaryl XB hosts by solvent-free ball-milling. The resulting selector-modied SWCNTs were then immobilized between gold electrodes and their conductance G measured in the absence and presence of pyridine (Fig. 36A).
As shown in Fig. 37, p-dihalobenzene selectors enabled sensing of low concentrations of pyridine gas (<25 ppm) by a decrease in electrical conductance, induced by swelling of the sensing matrix (Fig. 36B). Importantly, the p-diiodobenzene host enabled more sensitive sensing than the bromo or chlorocongeners, consistent with a sensor response arising from XBmediated recognition.
Specically, the p-diiodobenzene-containing sensor displayed the largest response of ÀDG/G 0 ¼ 5.1 AE 0.9% in response   to only 3 ppm pyridine, while much higher pyridine concentrations were required to induce signicant responses when the other selectors were employed. Further evidence for the crucial role of XB in this sensor was also obtained by experiments using iodo-or bromodurene as mono-haloaryl selectors as well as studies with 4-methylpyridine as analyte, which due to its enhanced Lewis basicity induced larger responses. Of further note is the response of these XB sensors displaying a relatively high level of selectivity; depending on the selector, even very high concentrations (>1000 ppm) of the potential interferents acetonitrile, benzene, isopropanol or hexanes induced only minor changes in conductance.
Similar chemiresistive gas sensors based on various p-dihalobenzene XB selectors and a SWCNT matrix were also developed for the detection of cyclohexanone and dimethyl-dinitrobutane (DMNB). 172 These analytes were chosen as model compounds for detection of nitro-containing explosives, the latter also serving as a tagant or marker compound in certain plastic explosives. Unsurprisingly, the diiodoaryl based selectors outperformed their bromo-counterparts for the sensing of cyclohexanone, with linear conductance decreases of up to 12%. Initial experiments indicate that the sensor can also detect the less volatile DMNB. A variety of other analytes also induced signicant responses (EtOH, ACN, EtOAc, cyclohexane and acetone), however this was attributed to their much higher vapour pressures and thus higher concentrations under the experimental conditions. Notably in both of these studies, the sensor was easily regenerated by exposure to pure N 2 gas, conrming the reversibility of the analyte-sensor interaction and enabling facile sensor re-use. 171,172 In 2016 Liu et al. reported a XB organogelator 31.XB containing multiple peripheral iodoperuoroarene moieties as a visual and rheological Cl À sensor (Fig. 38). 173 In acetone/hexane 1 : 8 the free gelator formed an organogel, which collapsed into a solution within 10 min upon addition of 0.8 equiv. Cl À . In contrast, the same amount of Br À only induced a small degree of gel-sol transition while HSO 4 À , NO 3 À , CN À or I À had no effect on the gel. Only at higher concentrations (2 equiv. for Br À and 5 equiv. for I À ) was dissolution of the gel achieved. These observations are in good agreement with the anion association constants of the gelator, determined by 1 H NMR titrations in acetone, which were largest for Cl À (650 M À1 ) and smaller for Br À (390 M À1 ) and I À (140 M À1 ), while the other anions did not bind signicantly.
A similar anion binding-induced transformation of a supramolecular assembly was also reported based on ChB quasi-calix [4]-chalcogenadiazole hosts 32.Te and 32.Se, which upon interaction with a pyridine N-oxide surfactant 33 in water selfassembled into vesicle or nanobers, respectively (Fig. 38). 174 Upon exposure to halide anions, or lowering the pH, these formations disassembled. In the case of the vesicles formed by 32.Te and 33, this was exploited as a proof-of-principle for release of the chemotherapeutic doxorubicin (DOX). Specically, DOX-loaded vesicles were ruptured by addition of Cl À or Br À , resulting in DOX release and increase of DOX uorescence, thereby presenting an indirect halide sensor.

Conclusions and outlook
The considered employment of sigma-hole interactions in the development of sensors, in particular for anions, but also neutral (gaseous) Lewis basic analytes, has signicantly matured in the last decade. This includes a large range of both optical and electrochemical sensing approaches, in particular those based on uorescent as well as voltammetric readouts. In addition, various other sensor formats, most notably chemiresistors as well as (redox)capacitive sigma-hole sensors have been developed recently. In all these formats an improved performance of the sigma-hole sensor (in particular XB) in comparison to a structurally analogous HB sensor is typically observed, including enhanced response magnitudes and sensitivities, lower LODs and/or enhanced/altered selectivity patterns. This arises as a result of enhanced binding magnitudes and/or enhanced signal transduction, which in turn can be attributed to the inherent characteristics of sigma-hole interactions, most notably lower solvent dependencies, higher hydrophobicities, stricter geometric binding preferences and altered thermodynamic binding contributions. As a result of these combined advantages, increasingly sophisticated and potent XB sensors have been developed, in particular in the last z5 years. This includes, for example, systems capable of anion sensing in increasingly competitive media, including pure water, 101,162,166 as well as continuous, real-time sensing systems. 71,72,131,171,172 These signicant advances in a comparably short amount of time attest to the enormous future potential of sigma-hole based sensors for real-life relevant applications and provide an excellent foundation for a broad range of research activities in sensor development and related applications as well as fundamental host-guest studies. We believe further efforts in this eld will/should focus on the following:

Sensing in aqueous media
In spite of the aforementioned examples, anion sensing in predominantly aqueous media remains a highly important but formidable challenge, which can, in part, be attributed to the signicant synthetic complexity of receptive, water-soluble probes. This can partially be circumvented by use of nondiffusive sensing formats (e.g. surface immobilisation) thereby negating the need for solubilising groups. Similarly, the omission of reporter groups can reduce synthetic complexity, but this can only be done if a sensing approach without a transducer is used, e.g. in potentiometric, chemiresistive or impedimetric/capacitive formats.

Fundamental studies into and exploitation of (other) sigma-hole interactions
While XB and, to a lesser extent ChB based sensors, are established, the application of pnictogen bonding (PnB) and tetrel bonding (TrB) as non-covalent supramolecular interactions for sensing has not been reported to date. Nevertheless, there is an increasing interest in the design and application of such receptors, 14,16,[175][176][177] and it is expected that they would display potent sensory performance. In light of the recently developed electroactive ChB systems 62 it appears that redox-control of PnB or TrB may be a particularly promising approach to not only generate novel sensors but to also gain fundamental insights into their intrinsic bonding properties. 177 Such fundamental investigations have already been carried out on a range of XB systems, in particular via voltammetric methodologies, 54,[125][126][127][128] which allow for the transient reversible generation of a differently charged (potentially not otherwise accessible) species. This enables simultaneous investigation of the sigma-hole properties of a receptor in multiple redox states, an approach that will undoubtedly prove useful in a continued investigation of sigma-hole properties.

Development of novel sensing approaches and mechanisms
We envision a further exploration of recently developed or (in the context of XB/ChB-mediated recognition) underexplored methodologies such as impedimetric or (redox)capacitive methodologies, 71,162 chemiresistive 171,172 or potentiometric approaches. 166 In addition, other sensing principles are ripe for exploration in concert with sigma-hole interactions, such as indicator displacement assays (IDAs) 178 or the recently developed transporter-liposome-uorophore (TLF) approach which relies on the quenching of a vesicle-encapsulated uorophore by the analyte ion. 179 In the latter, a selectivity enhancement is achieved by use of a transmembrane ion transporter, such that only ions which can cross the vesicle membrane and quench the encapsulated uorophore induce a response. Due to their typically improved or contrasting anion transport selectivities and efficacies, sigma-hole anionophores have much to offer as potent anionophores in TLF assays. [33][34][35][36]175,180

Device and materials integration
As highlighted in Section 2.3, sensor integration into condensed matter, in particular surfaces, membranes and polymeric architectures will be an indispensable avenue towards enabling many real-life relevant sensing applications such as ow sensors and microuidic devices, as only in these formats the main advantage of the non-covalent sensing approach, its reversibility, can be exploited. 68,71,72,181 In addition to sensor reusability and more facile device integration, this is associated with various other benets, including surface enhancement effects, circumventing solubility constraints, and the use of otherwise inaccessible sensing formats/readouts (e.g. impedance/capacitance/chemiresistance). 40,137,151 In spite of the enormous potential, surface immobilisation or material-integration sigma-hole mediated sensing remains comparably underdeveloped, and is only established in electrochemical 57,71,72,137,152,162 and chemiresistive formats. 171,172 Interfacial XB or ChB optical sensors remain even more embryonic; the only example being the benzoselenadiazole bres developed by Che for uorescent gas sensing. 131 Clearly there is signicant untapped potential in further exploration of XB and ChB (sensing) materials, 182,183 particularly in thin lms and (interfacial) polymers [184][185][186] in electrochemical, optical and other sensing formats.

Author contributions
RH and PDB co-wrote/edited the review article.

Conflicts of interest
There are no conicts to declare.