Luminescent chemosensors based on silica nanoparticles for the detection of ionic species

Abstract

The field of nanoparticles is very complex and many-sided, and these versatile materials find applications in different areas going from industry, to bio-analysis, and catalysis. Within this wide framework we have focused our attention on luminescent silica nanoparticles able to act as sensing materials. We present here an overview on recent examples having ionic species as the target analyte. Our analysis tries to highlight the huge potentiality offered by these nanoarchitectures that can allow pairing and even improving many of the typical features of molecular probes in several applications.

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Dr Marco Montalti obtained his PhD in Chemical Sciences in 2001 at the University of Bologna. In 1999 he was a research assistant at Tulane University (LA, USA); since 2002 he is an assistant professor at the University of Bologna. His research interests are in luminescent supramolecular systems and nanoparticles for sensing and energy processing.

Dr Enrico Rampazzo completed his PhD (2005) at the University of Padua (Italy). He is now a postdoctoral fellow at the Photochemical Nanosciences Laboratory (Bologna). His research interests focus on sensors and labels through the synthesis of luminescent self-organized systems based on silica nanoparticles.

Dr Nelsi Zaccheroni is an assistant professor in Chemistry at the University of Bologna since January 1999. She obtained the Chemistry degree and the PhD in Chemical Sciences at the same University in the Laboratory of Photochemistry and Supramolecular Chemistry. She spent one year as a postdoctoral fellow within a TMR-CEE project at the University College of Dublin (Ireland). Her research interests include luminescent systems for imaging and sensing, and the design and preparation of different nanoparticles.

Professor Luca Prodi received his PhD in 1992. He was appointed as a researcher in 1992 and an associate professor in 2004, in 2006 he was promoted to a full professor of General and Inorganic Chemistry at the University of Bologna. He was a visiting scientist at Argonne National Laboratory (IL, USA). His research activity is focused on the design of luminescent labels and sensors.

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Introduction

The widespread and increasing application of fluorescent chemosensors^1–3 naturally brings about the need for more and more efficient systems. Luminescent sensing represents, for example, one of the most rapidly developing fields in biology and medicine, where molecular probes are often the most useful tools for in vitro and in vivo monitoring of biologically relevant analytes. A fluorescent chemosensor is obtained by merging two fundamental moieties: the recognition site (receptor) and the signalling source (fluorophore) able to signal the binding event. These species, however, to provide an efficient detection, must meet many requirements such as, to cite only the main ones, a high selectivity or specificity for the relevant target and a stable luminescent signal (high photo-chemical stability) not affected by environmental interferences (for example physiological conditions).²

The advent of nanotechnology has opened up a number of new possibilities^4–6 to reach these goals overcoming many of the typical limitations of conventional systems. Among the newly available nanostructures that include Quantum Dots (QDs) and gold nanoparticles, luminescent silica nanoparticles (SiNPs) play, in our opinion, a very promising and valuable role.⁷ They can, in fact, potentially meet the crucial features for their application in many fields, including sensing.^2,7–13 To enter more into details, the features offered by this kind of NPs that can be used also for sensing are the following: (i) silica is photophysically inert, i.e., cannot be involved in quenching or photodecomposition processes; (ii) it does not present intrinsic toxicity, although a deeper investigation is still necessary to assess suitability for in vivo applications of SiNPs; (iii) each silica nanoparticle can contain a large number of photochemically active species allowing bright luminescence and collective processes; (iv) the silica matrix protects the active units segregated inside the nanoparticle, potentially increasing their photostability and fluorescence quantum yield, inferring water solubility. Moreover, luminescent SiNPs can be synthesized via different, simple, and versatile methods that allow the design of luminescent NPs with specific suitable properties for each application.^2,7

This favours the quite straightforward preparation of ratiometric systems by inserting a fluorescent probe together with an analyte-independent fluorophore into the silica structure, pursuing the final goal of sensitive and quantitative determination. The most common designs aim to prevent electronic interactions among the fluorescent units, thus eliminating the possibility of energy transfer processes. Typically, the reference dye is included in the core of the nanoparticles, while the probe is inserted in an outer shell or bound to the surface.^4,14 In other cases inter-chromophoric interactions become, in contrast, essential to obtain valuable functions such as signal amplification effects. This feature yields probes with an extremely high sensitivity since each single recognition event induces the photophysical response of many fluorescent signalling units at the same time. Energy transfer processes can be also designed to obtain remarkable Stokes shifts that typically allow a dramatic increase in the signal-to-noise ratio for more sensitive and precise photoluminescence measurements.

Moreover, the organization of the receptors into or on the silica structure can lead to a large increase in the affinity and/or selectivity towards the target analyte, thus improving the performance of the system.

We will discuss hereafter some selected examples (mainly from 2007 onward) of chemosensors for ionic species based on luminescent SiNPs. Even if this is only a limited subject in a very wide scenario, it still includes a huge amount of scientific research and literature. Cationic and anionic species, in fact, have a great impact on biology, medicine and on the delicate equilibria of the environment. The fine regulation of ionic species is vital and, consequently, their quantitative detection represents a crucial point in chemical research. For the aim of clarity, we have organized the following discussion in segments according to the different target analytes.

Silica NPs for the detection of cationic species

pH sensors

After the pioneering work of Wiesner and coworkers,¹⁵ several examples of fluorescent NPs designed for pH sensing have been reported. This is for example the case of the nanosensor developed by Doussineau and coworkers,¹⁶ who prepared NPs (circa 100 nm diameter) containing rhodamine as a reference and a fluorescent naphthalimide derivative grafted to the NPs surface through linkers of different length. The two-dye structure showed a good pH sensitivity in a physiologically relevant pH range with an ON–OFF response of the naphthalimide units.

A similar ratiometric system, able to sense pH variations in a range between 4 and 7 in murine macrophages and in living human cancer cells (HeLa cells) during apoptosis, was reported by Wang and coworkers.¹⁷ In this case smaller NPs (average diameter 42 nm) containing the tris(2,2′-bipyridyl)ruthenium(II) complex as reference and a fluorescein derivative as the luminescent sensing species showed good chemical and photochemical stability. They were incubated with murine macrophages and the changes in lysosomal pH were monitored in real time after exposure to the antimalarian drug chloroquine. Ratiometric fluorescence detection allowed the authors to conclude that chloroquine stimulates lysosomal pH changes. Upon incubation of HeLa cells with the same nanosensors it was instead possible to observe, in real time, intracellular acidification in apoptotic cancer cells after treatment with dexametasone, a synthetic glucocorticoid commonly used as an anti-inflammatory agent.

The pH of HeLa cells was also mapped with another ratiometric pH-sensor designed by Chen and coworkers¹⁸ using mesoporous silica NPs. Two fluorophores, fluorescein as the pH sensitive dye and rhodamine B as the reference dye, were grafted to the SiNPs. These authors used two differently charged ethoxysilane derivatives to functionalize the NPs surface and observed how the charge influenced the final location of the NPs in the cells. In particular, positively charged NPs were found to prefer higher pH regions (mostly cytosol) while negatively charged NPs accumulated in acidic endosomes. Interestingly, evidence was given for direct penetration of membranes through a charge-mediated membrane–NP interaction mechanism.

A different approach to ratiometric pH detection uses rhodamine lactams, nonfluorescent compounds that form highly fluorescent species via analyte mediated opening of the lactam form (cyclic amide).¹⁹ In this case, ratiometric sensing of lysosomal pH in live cells was achieved with dual coloured mesoporous silica nanoparticles (diameter of 100 nm) containing acid activable rhodamine 6G lactam (R6G-lactam) and fluorescein that, inversely, exhibits decreased fluorescence in acidic media. Notably, the inverse pH response of the two dyes leads to much larger changes in the ratio between the two emission intensities, and thus to a much higher precision in pH determination.

Copper sensors

Jung and coworkers reported two examples of nanosensors for copper(II) derivatizing the surface of the silica NPs (diameter of ca. 15 nm) with specific receptors bearing alkoxysilane groups. In the first case²⁰ the sensing molecule was a phenanthroline derivative and the resulting system showed high selectivity for copper, the only species that caused a strong fluorescence quenching even in the presence of a large excess of other metal cations. In the second example²¹ the recognition was achieved with a trialkoxysilane derivative of fluorescein bearing two coordinating sites and these capped NPs were used to detect Cu²⁺ in living cells. The nanosensor, in fact, had shown a selective and reversible (upon addition of EDTA) response to copper concentration in water at pH 7.4, and it resulted in ability to permeate into HeLa cells and to detect the presence of Cu²⁺ also in this environment.

We decided to exploit a similar approach based on the covalent binding of the molecular chemosensor to the nanostructure but, in our case, including them inside the same silica NP rather than appending them to the surface. We tested this approach with the aim to obtain signal amplification with a consequent improvement in the response sensitivity upon complexation of the target analyte. The NPs (30 nm) were synthesized with the Stöber method in the presence of a high concentration of a dansylated commercial receptor, the 3-[2-(2-aminoethylamino)ethylamino]-propyl-trimethoxysilane,²² as a chemosensor. In EtOH–H₂O 2 : 1 v/v solutions, the addition of copper(II), cobalt(II) and nickel(II) ions caused a strong quenching of the fluorescence intensity of these NPs even at nanomolar concentrations. In particular, titration experiments demonstrated that a single copper ion was able to quench up to 13 dansyl units, largely improving the sensitivity of the molecular chemosensor thanks to the induction of cooperative effects associated with energy and electron transfer processes.

Similar results were obtained by Mancin and coworkers with a more flexible approach exploiting the intermolecular interactions of a separate fluorophore–receptor pair bound on the NPs.²³ The self-assembly of the receptor, a picolinamide, and of the signalling unit, a dansylamide, on the surface of preformed silica nanoparticles (20 nm diameter) ensured the required proximity for signal transduction (fluorescence quenching) and the NPs were able to detect Cu²⁺ ions down to micromolar concentrations in a selective way. The main advantage of this approach is its high modularity that can lead to an easy preparation of structures selective for different analytes through a proper combination of the components.

Zong et al.²⁴ preferred to exploit a more classical ratiometric approach and synthesized a dual-emission fluorescent SiNPs-based nanosensor for rapid and ultrasensitive detection of Cu²⁺ (Fig. 1). These authors used the signal of fluorescein-doped silica cores as reference and covalently grafted a rhodamine on the surface of the NPs through a chelating reagent for Cu²⁺. The selective quenching of the rhodamine molecules by copper ions led to an orange-to-green colour switch of the emission clearly observable also with the naked eye. A detection limit as low as 10 nM was reached with this nanoprobe, a level about one thousand-fold lower than the allowed Cu²⁺ limit in drinking water.

Fig. 1 Schematic representation of the copper complexation and the consequent fluorescence ratiometric signal of core–shell NPs. Adapted with permission from ref. 24. Copyright © 2011, American Chemical Society.

Also copper(I) is a very interesting species, playing a role, among the other, in neurodegenerative diseases. In this framework we have developed a ratiometric nanosensor exploiting the synergistic contributions of the specific chemosensor designed by Chang et al.²⁵ and of core–shell silica NPs (Fig. 2).²⁶

Fig. 2 Mechanism of signal amplification proposed for core–shell NPs. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Adapted from ref. 26.

The NPs were synthesized by using a micellar system (Pluronic F127) as a template. Thanks to this approach, the formation of the silica core (containing a coumarin derivative as reference dye) blocks the surfactant branches to form an organic PEG shell suitable to host the molecular chemosensor units. The amplification of the fluorescence response to complexation, due to cooperative energy transfer from the coumarin dyes present in the silica core to the bodipy moieties hosted in the outer shell, was evidenced in this nanosystem. This is a simple but general approach to achieve amplification in ratiometric nanosensors that, interestingly, present two additional advantages compared to molecular probes. This system ensures a low noise presenting a high separation between the excitation and emission wavelengths. Moreover, the chemosensor inside the PEG shell shows a ten-fold increase in the association constant with Cu⁺ ions, allowing lower detection limits in biological environments. Interestingly, this approach enables the use of otherwise water insoluble chemosensors, increasing the number of their possible applications.²⁷

Zinc sensors

Rastogi et al. presented an interesting fluorescent nanosensor for Zn²⁺ based on a derivative of 8-aminoquinoline grafted on silica NPs²⁸ (Fig. 3) and demonstrated its application in tap water and in suspensions of yeast cell (Saccharomyces cerevisiae). The nanosensor showed, upon zinc complexation, a 2.8-fold fluorescence enhancement and a 55 nm red-shift, no interferences by other metal ions and a submicromolar detection limit under the above mentioned conditions.

Fig. 3 Schematic representation of fluorescent SiNPs for zinc detection. Adapted with permission from ref. 28. Copyright © 2011, American Chemical Society.

Mancin and coworkers used a similar metal sensitive probe (TSQ, 6-methoxy-(8-p-toluenesulfonamido)quinoline), co-polymerized with TEOS, to prepare a ratiometric system for zinc detection, with coumarin derivatives as reference fluorophores.¹⁴ TSQ is a well known OFF–ON fluorescent chemosensor that binds zinc ions with a good selectivity. The system was able to detect zinc ions in water solutions with a submicromolar sensitivity (0.13 μM) with the only interference of Cu²⁺ and Cd²⁺. Together with Mancin and coworkers, we decorated the surface of commercial silica nanoparticles, using TSQ derivatized with a silane, to obtain a fluorescent chemosensor for Zn(II) with an amplified OFF–ON fluorescence response to complexation (Fig. 4).^7,29

Fig. 4 Representation of OFF/ON amplification processes in multichromophoric SiNPs. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Adapted with permission from ref. 7.

In particular, at low zinc concentration this self-organized nanosensors gave a signal increase 50% higher with respect to the reference system TSQ under the same conditions. We have to underline that these latter NPs were designed to have a very short interchromophoric distance, leading to a higher efficiency of the energy transfer process that is crucial for signal amplification. Moreover, the pre-organization on the surface of the nanoparticles of the sensing units leads to an enhanced affinity toward the target ions (the association constant increases of circa four orders of magnitude in the NPs), resulting in a great increase in the system sensitivity.

Lead sensors

Dansyl doped NPs were used by Mancin et al.³⁰ to detect Pb²⁺ by fluorescence quenching. In a first approach the surface of 50 nm silica nanoparticles was functionalized with 3-mercaptopropyltriethoxysilane (MPS). The resulting system was water soluble and showed photophysical properties typical of the dansyl dye. Although fluorescence quenching was observed after the addition of lead ions to buffered (pH 7) solutions, a residual 60% fluorescence was detected even at very high Pb²⁺ concentrations. The same authors, then, proposed improved systems to overcome this drawback. They first prepared a set of core–shell NPs with a 50 nm nucleus of pure silica surrounded by a 5 nm-thick silica shell doped with dansyl moieties, and capped with MPS. In this case the residual fluorescence was reduced to 30% as a result of the decreased average distance between the fluorophores and the bound metal ions. In a third option, a multishell system was prepared made of a silica core doped with a reference coumarin derivative covered by an insulating pure silica layer and finally by an outer dansyl doped silica shell capped with MPS. This last system did not show improved performance with respect to the single-shell one but allowed ratiometric detection and calibration.

Jung et al.³¹ made a step forward addressing also another important goal: lead separation in different matrixes. They grafted a bodipy derivative onto the surface of nickel-core–silica-shell NPs (30–40 nm diameter). This dye, buffered at pH 7, undergoes an internal photoinduced electron transfer (PET) process that quenches its fluorescence. Under these conditions lead ions cause a 8-fold fluorescence enhancement at 510 nm, an effect that is reversible with the addition of a strong base. The high selectivity for Pb²⁺ over other possible interfering cations was achieved due to the preorganization of the chemosensor on the silica surface, which yields in a very sensitive probe with a detection limit lower than 15 ppb (the maximum limit allowed for lead in drinking water). Thanks to its metal core, magnetically driven lead extraction experiments were also performed on human blood allowing to remove 96% of the total 100 ppb Pb²⁺ content, which is the lower unsafe limit in children blood. The same authors proposed also an improved system using another bodipy derivative in similar silica NPs.³² An overall emission change of ca. 100-fold at the emission maximum was observed upon lead complexation. All these experiments showed that this kind of NPs could be a potentially useful and effective agent for the identification, selective separation, and rapid removal of Pb²⁺in vivo.

Mercury sensors

The presence of mercury ions in aqueous solutions is a main environmental concern that has been addressed by many researchers. Chen and coworkers, for example, have proposed a ratiometric fluorescent system for their detection in water.³³ This multilayered nanosensor is made of a silica NP (100 nm diameter) surrounded by a shell of CdTe QDs embedded in a silica matrix. Dual emission is achieved by capping this nanospheres with a silane derivative of rhodamine 6G. The first fluorescence band at 545 nm (rhodamine) is pH dependent, while the second one at 625 nm (QDs) is not. In particular, in PBS buffered solutions the rhodamine is present in the lactam form that, as already discussed, is not fluorescent. Under these conditions the presence of Hg²⁺ in solution induces a strong emission enhancement concomitant with the ring opening of the lactam, comparable to protonation. This complex chemosensor offers high selectivity toward Hg²⁺ and a low detection limit (down to the nanomolar concentration).

Sometimes, above all in biological and medical applications, the probe dimension is a critical feature. García-España, Alarcón and coworkers³⁴ have reported another core–shell system of much smaller dimensions (5–10 nm diameter) based on an anthracene derivative and simple secondary amines as receptors. An aluminosilicate core is surrounded by a silica shell functionalized with the active units. The authors observed a specific fluorescence quenching in the presence of Hg²⁺ in the 3.5–5.5 pH range, with a detection limit in water of 0.2 ppb. Moreover, the material gelification at basic pH allowed its simple recovery by centrifugation.

Jung et al., as for the case of lead, entered the field with the dual aim of detection and extraction of Hg²⁺.³⁵ They prepared Fe₃O₄-core–silica-shell NPs (ca. 20 nm, narrow size distribution) functionalized with aminonaphthalimide units at the surface, resulting in a sensitive and selective chemosensor for mercury and methylmercury ions in the pH range of 4–11. The presence of the magnetic 4 nm Fe₃O₄ nanocore also allowed the very efficient removal of these toxic agents from drinking water (containing both at the 100 ppb level) using an external magnetic field.

Also Wu and coworkers proposed a system for Hg²⁺ sensing in water, but they took advantage from the multilayered structure of SiNPs to obtain a FRET mechanism for a ratiometric detection (Fig. 5).³⁶ In particular, a nitrobenzoxadiazolyl derivative was covalently confined into a thin layer of these particles, while a spirolactame rhodamine derivative was covalently linked onto the NPs surface; the first component could act as the donor for the mercury ion probes at the surface. This kind of architecture allowed a good control of the spatial distribution of both donor and acceptor units and of their mutual separation distance within each nanoparticle. As a consequence, high energy transfer efficiency and signal-to-noise ratio were achieved.

Fig. 5 Schematic illustration of the formation of a FRET-based ratiometric Hg²⁺ sensor with multilayered silica NPs as the scaffold. Adapted from ref. 36 with permission from the Royal Society of Chemistry.

Silica NPs for the detection of anionic species

The design of sensitive and selective fluorescent sensors for anions is generally a much more difficult task than the preparation of cation probes. The main reason is that anions have typically a wider range of geometries and a lower charge-to-radius ratio than cations, features that require higher complexity in the design of the receptors.³⁷

The group of Martinez-Manez, particularly active in this field, developed original systems exploiting self-assembling of anthracene and different thiourea–silane derivatives on commercial silica nanoparticles (18 nm diameter).³⁸ In particular, the authors investigated the effects of the composition of the nanosensors, of the synthetic strategy and of the nature of the ligands on the recognition of organic (acetates, benzoates) and inorganic (phosphates, sulphates and halogenides) anions. Although none of the proposed materials showed significant selectivity, and only moderate fluorescence changes were observed in the presence of anions, these systems can be seen as an important proof of principle for this very interesting modular approach that requires a relatively low synthetic effort.

More recently, the same authors made a further step forward proposing the use of SiNPs bearing sulforhodamine B as a fluorogenic signalling unit, and either imidazolium or guanidinium binding groups as anion coordination sites.³⁹

The NPs were prepared by the simultaneous grafting of suitable derivatives of the two subunits on the same commercial silica nanoparticles. Acetonitrile suspensions of the NPs showed the typical photophysical characteristics of sulforhodamine B. They studied the effects of the presence of organic (acetates, benzoates) and inorganic anions in acetonitrile and in acetonitrile : water 9 : 1 v : v solutions. Interestingly, imidazolium based NPs showed a fluorogenic response toward iodide and benzoate, whereas guanidinium funtionalized NPs selectively responded to dihydrogenphosphate and hydrogensulphate ions.

Finally, the same group, together with Prof. Rurack, synthesized a sensing ensemble by grafting sulforhodamine B and terpyridine onto the surface of SiNPs. Coordination of some metal ions (Fe³⁺, Hg²⁺, Cu²⁺, Ni²⁺, and Pb²⁺) at the terpyridine moieties induced the quenching of the dye fluorescence in acetonitrile. The addition of anions resulted in their metal coordination signalled by a partial fluorescence recovery, dependent both on the metal ions and the added anion. This allowed differential recognition of small inorganic anions in a quencher displacement assay.⁴⁰

Bau et al. prepared a ratiometric probe for chloride anions based on NPs, which are able to penetrate neuronal cells at submillimolar concentrations.⁴¹ These SiNPs (Fig. 6) were grafted with 6-methoxyquinolinium as the chloride-sensitive component and fluorescein as the reference dye. A Stern–Volmer constant of 50 M⁻¹ was measured in a chloride-free Ringer's buffer at pH 7.2, and the response to chemically induced chloride currents was recorded in real time in hippocampal cells.

Fig. 6 Structure of a fluorescent ratiometric nanosensor for chloride ions. Adapted with permission from ref. 41. Copyright © 2011, American Chemical Society.

Dipicolinc acid (DPA) anions are not a biologically relevant component but their sensing is attracting attention for safety reasons since it is a marker of bacterial spores (for example B anthracis spores). Lin and Taylor have developed a ratiometric system for its detection based on SiNPs containing Tb(III)⁴² ions. DPA is a known ligand for lanthanide ions, able to efficiently sensitize their emission. This energy transfer process leads to a very large luminescence increase upon complexation and this was exploited by the authors to obtain DPA quantification. The nanosensor was prepared by functionalizing the surfaces of SiNPs with modified terbium EDTA complexes bearing one or two alkoxysilane groups. For taking benefit of the use of an internal reference signal, the authors used SiNPs doped with the tris(2,2′-bipyridyl)ruthenium(II) complex. A quite specific linear increase in the terbium luminescence was observed as a function of DPA concentration with a detection limit in the nanomolar range.

A second nanosensor for DPA, but based on an europium complex, was reported by Lu and coworkers.⁴³ The fluorescein (as reference dye) doped silica nanoparticles (65 nm diameter) were derivatized at the surface with silanized ethylenediaminetetraaceticdianhydride (EDTAD) europium complexes. These NPs undergo fluorescence enhancement in the presence of DPA via a transduction mechanism analogous to the one described above for Tb derivatized systems, but presenting a larger Stoke shift and a red shifted emission. The luminescence switch of the NPs is proportional to DPA concentration in the 0.6–600 nM range with a detection limit as low as 0.2 nM.

Conclusions and perspectives

Recent developments in the design and preparation of nanosized materials have yielded powerful and versatile platforms for addressing crucial issues in sensing, imaging, and molecular testing. In this context we have selected some examples trying to evidence the many advantages and the most valuable features of luminescent multichromophoric silica-based nanoprobes. These materials are very promising offering very high monodispersity, remarkable stability and solubility in water even under physiological conditions, combined with low intrinsic toxicity. Their high synthetic versatility also offers the possibility to design multilayered structures, to obtain collective energy- and electron-transfer processes. These phenomena are at the base of highly valuable features, such as signal amplification, that still remain largely unexplored.

To conclude, the great added value of SiNPs can be summarized by two concepts: modulation and multifunctionality. Their combination, that can be obtained following a rational and proper engineering, can yield the contemporary optimization of many aspects of the whole performance, such as sensitivity, reproducibility, affinity, signal amplification, and internal calibration. This already makes fluorescent SiNPs a unique platform in the nanotechnology arena, but we strongly believe that most of their potentialities have not yet been fully developed. A natural implementation will be, for example, the multiple detection of several analytes at the same time with the same particle. Moreover, while separation and sensing have already been obtained with the same structure, in a longer time perspective the combination of analyte detection and drug delivery will represent a real breakthrough. Despite its difficulty, this task is already attracting the attention of many laboratories worldwide.

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Footnote

† This article is included in the All Aboard 2013 themed issue.

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