Tunable dielectric nanoarchitectonics in carbon nanotubes via DNA-directed Pd(II) nanoarrays

Abstract

The precise spatial organization of metal species on carbon nanotube (CNT) surfaces is crucial for tailoring their electronic and dielectric properties, yet remains difficult to achieve in a controlled manner. Here, we report a CNT-based nanohybrid in which one-dimensional Pd(II) arrays, templated by single-stranded DNA (dA₁₅), are covalently anchored onto carboxyl-functionalized multi-walled CNTs. Building on our previous demonstration of continuous helical Pd–DNA architectures, we translate this structural precision to CNT surfaces, yielding uniformly decorated nanotube scaffolds. Comprehensive spectroscopic and microscopic analyses (FTIR, UV–Vis, TEM/HRTEM, EDS) confirm successful assembly and nanoscale organization. Electrical impedance spectroscopy reveals that the resulting CNT–DNA–Pd hybrid exhibits a distinct, tunable dielectric response compared to its individual components. This approach provides a versatile strategy for integrating programmable DNA-based metal architectures with carbon nanomaterials for potential applications in electronics and energy-related devices.

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Introduction

Functionalization of carbon nanotubes (CNTs) is essential to unlock their full potential in a wide range of applications, including structural materials, (bio)sensing, electronics, energy storage, environmental remediation, and biomedicine.^1–7 By tailoring their surface chemistry, properties such as chemical reactivity, dispersibility, adsorption, catalytic performance, and electrical conductivity can be significantly enhanced. Over the past two decades, extensive efforts have been devoted to the development of both covalent and non-covalent strategies for decorating CNTs with diverse structural motifs.^8–13 Despite these advances, achieving a controlled and well-defined arrangement of molecules or metals on CNT surfaces is still challenging. In this context, we have been actively exploring new covalent functionalization methodologies for CNTs, employing a variety of functional groups and nanostructures including halogens, oxygen-containing moieties, hyperbranched polyamines, and palladium or platinum nanoparticles.^14–21 In particular, the precise spatial organization of metal species along the nanotube surface and the control of their oxidation states are of great interest, as they directly influence the electrical properties of the resulting nanocomposites.^22,23 Following this foundation, we have built upon previous studies demonstrating the use of DNA oligonucleotides as templates for ordering metal ions and metal complexes at the nanoscale.

The versatility of DNA as a programmable molecular system to generate novel nanoarchitectures, especially in combination with metals, attracts intense interest in nanoelectronics, optics, sensing and other cutting-edge fields.^24–33 In this context, the search for methodologies capable of yielding highly organized metal–DNA assemblies, where the metal ions presence endows DNA molecules with new chemical–physical–structural features (e.g., conductivity, fluorescence, or magnetism), becomes an area of intense research interest, aiming to exploit the extraordinary ability of DNA to rationally interact with metallic matter at the nanoscale and create new molecular systems.³⁴ We previously demonstrated that Pd(II) complexes can be organized along single-stranded DNA molecules (ssDNA) in a controlled manner, selectively occupying Watson–Crick nucleobases recognition sites.^35,36 In these assemblies, the metal ions form a continuous one-dimensional arrangement dictated by the DNA template, positioning themselves in a precise linear atom-to-atom fashion. Moreover, the ligands coordinated to the Pd(II) centers follow the same chiral trajectory, giving rise to a well-defined Pd–DNA supramolecular helical architecture.

Guided by these results, we envisioned translating the precise one-dimensional Pd–DNA nanoarrays from solution to the surface of CNTs. In doing so, our goal is not only to uniformly organize the Pd(II) centers helically around the nanotube exterior, but also to introduce a controllable supramolecular interface capable of modulating the dielectric behavior of the CNTs. By using DNA as a programmable template for Pd(II) positioning, we seek to create tunable CNT–DNA–Pd nanoarchitectures in which the electronic environment of the nanotube can be rationally tailored. This strategy enables CNT functionalization through the precise structural control and chiral arrangement offered by DNA-templated Pd(II) arrays, leading to the formation of well-defined, multicomponent nanohybrids. Previous electrochemical impedance spectroscopy (EIS) studies on metal–DNA systems have shown that metal coordination can significantly modify the charge-transfer properties of DNA layers.^27,37,38 However, these effects have been explored almost exclusively in surface-bound films and remain largely unexamined in dispersed hybrid nanostructures. Another study using single-molecule break-junction measurements with SWNTs has shown that metal–DNA systems can enhance charge transport through DNA.³⁹

For our work, we selected multi-wall carbon nanotubes functionalized with –COOH groups (CNT–COOH), which were covalently coupled to a 15-adenine oligonucleotide functionalized with terminal amino group at the 3′ end (dA₁₅). This DNA-functionalized CNT scaffold then serves as the template for the self-assembly of the coordination compound [Pd(Cheld)(CH₃CN)] (Cheld = chelidamic acid) (1Pd), as previously reported by our group.^35,36 The synthetic sequence used to prepare the CNT–dA₁₅–Pd nanohybrid is illustrated in Fig. 1. To investigate the dielectric of these supramolecular assemblies, we characterized the individual components and their corresponding hybrids using EIS, and the resulting data were analyzed with an equivalent-circuit model.

Fig. 1 Schematic representation of the formation of CNT–dA₁₅–1Pd hybrids and its electronic properties trends.

Results and discussion

Covalent attachment of oligonucleotide dA₁₅ to carbon nanotubes (CNT–COOH)

Carboxyl groups on the surface of CNT–COOH served as anchoring points for a coupling reaction with the terminal amino group of dA₁₅, forming amide bridges between the nanotube surface and the oligonucleotide to yield the CNT–dA₁₅ compound. The attachment reaction was monitored using UV-vis spectroscopy registered at different times until no variances were observed. The comparison of the UV-vis spectra at the start (t₀) and end (t_f) showed a decrease in the band center at 260 nm, corresponding to the absorption of dA₁₅, in agreement with a decrease in concentration due to its incorporation into the carbon nanotubes (Fig. S2). The solid was then centrifuged and subjected to a quick wash to remove unreacted oligonucleotide left free in solution. The solid was then analyzed by means of XPS and the detection of P atoms in the wide spectrum of the CNT–dA₁₅ hybrid (Fig. S3) unequivocally confirms the presence of dA₁₅. Table 1 summarizes the surface composition of the different samples. The P atomic concentration in CNT–dA₁₅, in conjunction with the molar mass of dA₁₅ (4773.26 g mol⁻¹), has allowed for the calculation of a substantial dA₁₅ content of ≈15 wt%. Furthermore, the increases in O and N in CNT–dA₁₅ compared to CNT–COOH, +3.2 and +4.1 at%, respectively, are consistent with the detected amount of dA₁₅ and its stoichiometric formula, C₁₅₃H₁₈₉N₇₆O₇₆P₁₅.

Table 1 Surface element composition (XPS) of pristine CNT and modified materials

Sample	XPS concentration (at%)^a/(wt%)^b
	C	O	N	P
a Surface atomic percentages derived from C 1s, O 1s, N 1s and P 2p region areas in the wide spectra (see XPS wide spectra for all the samples in Fig. S3).b Surface mass concentrations calculated from the corresponding atomic values.
CNT	98.9/98.6	1.1/1.4	—	—
CNT–COOH	91.7/89.2	8.3/10.8	—	—
CNT–dA15	83.9/79.6	11.5/14.5	4.1/4.5	0.6/1.5

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High-resolution XPS spectra further confirms the presence of dA₁₅ in the CNT–dA₁₅ hybrid. Fig. 2a compares the normalized C 1s profiles of the three samples. The CNT–COOH profile shows an increase in intensity around 288.5 eV, indicating its –COOH functionalization.⁴⁰ In the CNT–dA₁₅ profile, a significant increase is observed from ≈285 eV to ≈289 eV, corresponding to C atoms bonded to O or N,⁴¹ i.e., the carbon backbone of dA₁₅. See C 1s deconvolutions for each sample in Fig. S4. Additionally, the N 1s peak deconvolution of CNT–dA₁₅ (Fig. 2b) reveals two components that can be related to N atoms in the purine skeleton (399.1 eV) and amino groups (400.4 eV) in the adenine base.^42,43 This last component can be also related to the N atom of amide bridge formed between dA₁₅ and the nanotube surfaces.⁴⁴ The [3/2] component at 133.2 eV in P 2p peak deconvolution (Fig. 2c) is attributable to the phosphate groups in dA₁₅.^45–47

Fig. 2 (a) XPS high-resolution C 1s regions of CNT, CNT–COOH, and CNT–dA₁₅. Deconvolutions of the XPS high-resolution, (b) N 1s and, (c) P 2p core-level peaks for CNT–dA₁₅, (d) HRTEM micrograph and EDS element distribution maps of C, O, N and P for the CNT–dA₁₅ hybrid, (e) IR spectra of CNT, CNT–COOH, and CNT–dA₁₅, and comparison of CNT–dA₁₅ and adenine reference IR spectra. Blue dots mark peaks common to both spectra.

High-Resolution TEM and elemental mapping (HRTEM/EDS) further support immobilization of dA₁₅ on CNT surfaces, as shown in Fig. 2d, where N and P are co-localized along the nanotube contour (additional maps and EDS spectra in Fig. S5 and S6). FTIR results displayed in Fig. 2e, two prominent bands in the CNT–dA₁₅ spectrum (located at 1640–1680 cm⁻¹) stand out, attributable to the carbonyl stretch vibration within the amide group,⁴⁸ consistent with formation of covalent anchoring of dA₁₅ to the nanotubes. Additionally, comparing the spectrum of the hybrid material with the adenine reference spectrum (NIST)⁴⁹ in Fig. 2f reveals common IR features, supporting the presence of the oligonucleotide. Additionally, the observation of two weak bands at ≈2850 and ≈2930 cm⁻¹ in (Fig. S7) has also been described as indicative of organic macromolecules of a biological nature, such as DNA and proteins.⁴⁸

Preparation of CNT–dA₁₅–1Pd hybrid systems

After connecting dA₁₅ onto the nanotube surface, the 1Pd complexes were subsequently assembled along the DNA strand, adding a new level of structural organization to the hybrid. The self-assembly of 1Pd with dA₁₅ single strands has been reported previously, and the same procedure was applied in this study to obtain the multicomponent CNT–dA₁₅–1Pd hybrid.³⁶ An aqueous dispersion of solubilized CNT–dA₁₅ was titrated with increasing amounts of 1Pd and the reaction was monitored by means of circular dichroism (CD). Before adding 1Pd complex, the CD spectrum of CNT–dA₁₅ exhibited a weak and noisy signal, attributed to the intrinsic dispersity of the sample arising from the presence of carbon nanotubes (Fig. 3a). Upon incremental addition of 1Pd, an increase in ellipticity at 260 nm accompanied by a decrease at 300 nm is observed, and the CD profile progressively evolves into a more defined sinusoidal shape. These spectral changes are consistent with the interaction of 1Pd with dA₁₅ and the formation of a helical structure stabilized by palladium-mediated base pairs, in agreement with previous reports.³⁶ A control experiment performed with CNT–COOH, in the absence of dA₁₅, showed that the addition of 1Pd induces a ellipticity increase at 280 nm (Fig. 3b), likely arising from interactions between aromatic-planar 1Pd and the nanotube surface. However, these interactions do not produce the characteristic sinusoidal CD profile observed in the presence of dA₁₅. Therefore, the spectral changes in Fig. 3a can be attributed exclusively to the self-assembly of 1Pd on the oligonucleotide within CNT–dA₁₅. Furthermore, UV-vis spectroscopy (Fig. 3c) provides additional evidence of the 1Pd–adenine base interaction in CNT–dA₁₅–1Pd, as indicated by the shift of the maximum to shorter wavelengths, ≈275 nm, compared to the free complex, ≈300 nm.

Fig. 3 CD titration spectra for the reaction of 1Pd with (a) CNT–dA₁₅, and (b) CNT–COOH; the samples were UV-Vis normalized to similar optical density, excluding the 260 nm DNA band (note: 1 equivalent means one 1Pd per adenine base). (c) Comparison of the UV-vis spectra of 1Pd, CNT–dA₁₅, and CNT–dA₁₅–1Pd sample.

HRTEM/EDS measurements revealed the multicomponent architecture of CNT–dA₁₅–1Pd, with atoms of P, N, and Pd, indicating that dA₁₅ and 1Pd, are distributed along the carbon nanotube (Fig. 4 and Fig. S8–S10).

Fig. 4 HRTEM micrograph and EDS element distribution maps of P, Pd, C, O and N for the CNT–dA₁₅–1Pd hybrid.

Dielectric properties of CNT–dA₁₅–1Pd hybrid systems

The electrical properties of the CNT–dA₁₅–1Pd hybrids were evaluated by electrical impedance spectroscopy (EIS). Building on the spectroscopic evidence supporting the formation of the 1Pd–dA₁₅ assemblies on the CNT, we aimed to assess how their incorporation modulates the electronic properties of the CNTs. In particular, the well-defined one-dimensional Pd arrangement provided by the 1Pd–dA₁₅ system offers a controlled means to influence the dielectric response of the resulting CNT–dA₁₅–1Pd hybrid material. EIS measurements were carried out on aqueous solutions containing diverse samples. One solution contained the CNT–dA₁₅–1Pd hybrid, while the other two contained the free 1Pd complex and the 1Pd–dA₁₅ system, respectively. All solutions were prepared with the same concentration of the complex to avoid any influence due to concentration differences. Additionally, two control solutions were evaluated, both of which avoid the presence of 1Pd. One corresponded to the pure solvent (MOPS buffer and DMF), and the other contained CNT–dA₁₅. Since all samples were prepared in the same aqueous medium and measured under identical temperature and AC excitation conditions, the differences detected in their dielectric parameters can be attributed to the materials present in each sample.

Thus, EIS measurements were performed on five solutions (CNT–dA₁₅–1Pd, 1Pd, 1Pd–dA₁₅, CNT–dA₁₅, and solvent) together with the empty fixture used to determine the reference capacitance (C₀). Fig. 5a shows the Bode plots of the complex impedance, displaying its magnitude and phase versus frequency, while Fig. 5b shows the corresponding Nyquist plots (−Im(Z_eq) versus Re(Z_eq)). It should be noted that the impedances displayed in Fig. 5 (Bode and Nyquist) were extracted directly from measurements performed with the 16452A parallel-plate fixture, without additional post-processing or equivalent circuit fitting.

Fig. 5 (a) Real and imaginary part of the measured complex impedances in the frequency range between 100 Hz and 5 MHz. (b) Nyquist diagrams from 100 Hz to 5 MHz. (Note that solvent and CNT–dA₁₅ curves are almost fully overlapping in both plots.)

From Fig. 5a, we observe a null phase around 5 kHz, corresponding to pure resistive behavior and an increasing contribution of the capacitive term for higher frequencies. The semicircle in the Nyquist plot can be described by an equivalent circuit containing a resistor and a capacitor connected in parallel (Fig. S11a), while the straight line for frequencies below 5 kHz is explained by the mass-transport limitation to and from the electrodes, which can be modeled by the Warburg impedance connected in series with the resistance of the parallel circuit (Fig. S11b).⁵⁰ In the frequencies of interest, the magnitude of the impedance shows a noticeable dependence on the composition of the solution. Firstly, the CNT–dA₁₅ solution is indistinguishable from the solvent, indicating that both CNTs and dA₁₅, in the absence of 1Pd complex, have no influence on the resulting impedance.

For the 1Pd–dA₁₅ and 1Pd solutions, a significant reduction in impedance was observed in both cases, indicating that the self-assembly of 1Pd into a nanoarrangement along the dA₁₅ chains has no significant effect on the dielectric properties of the samples.

However, the attachment of 1Pd complexes to the CNT–dA₁₅, which a priori does not affect the impedance (as previously observed for the CNT–dA₁₅ solution), results in an increase in the measured impedance compared to the 1Pd–dA₁₅ and 1Pd solutions, indicating that the organization of the complexes along the ss-DNA template within the CNT–dA₁₅–1Pd hybrid increase the resistance.

From the perspective of the CNT–dA₁₅ hybrid, which is incapable of altering the impedance of the resulting solution by itself, it is interesting to observe that the incorporation of 1Pd does lead to a substantial decrease in impedance from ∼900 Ω up to ∼700 Ω (see Fig. 5b). The impedance changes observed upon metal coordination indicate that dA₁₅–1Pd interactions can markedly modulate the electrical response of the CNT–dA₁₅–1Pd hybrid. While our measurements probe the effective (bulk) response of the dispersion, previous impedance studies on surface-bound DNA architecture have likewise shown that the presence and binding mode of metal ions can alter the measured electrical properties in a metal-responsive manner.^38,51 Accordingly, without implying a direct quantitative comparison between these distinct experimental configurations, our results support the general notion that metal coordination to DNA provides an efficient handle to tune the electrical behavior of DNA-based materials.

Employing the parallel plate capacitance (C_p) and resistance (R_p) extracted, together with the reference capacitance (C₀), the complex dielectric constant is evaluated by means of the test fixture⁵² and eqn (1)–(3). These equations assume that the R_pC_p model accurately captures the behavior of the SUT (as proved in Fig. S12); therefore, the corresponding results and assumptions are valid for frequencies above 5 kHz.

Fig. 6a shows that at low frequencies the real part of relative dielectric constant () extracted from the ratio between parallel and reference capacitance, can be grouped in two groups depending on whether the self-assembly of 1Pd along dA₁₅ is present in the solution or not. Thereby, the interaction of the 1Pd complex with the dA₁₅ strands (regardless of whether it is CNT–dA₁₅–1Pd or 1Pd–dA₁₅, that is, regardless of the immobilization of dA₁₅ chains on the nanotubes) can be identified as the main responsible for the decrease in parallel capacitance, and therefore, for the lower relative dielectric constants. Later, from ∼20 kHz frequency onwards, all samples converge to the value established by the solvent.

Fig. 6 (a) Relative dielectric constant in the frequency range between 100 Hz and 5 MHz. (b) Dielectric losses in the frequency range 100 Hz–5 MHz.

As for the dielectric losses () depicted in Fig. 6b, the trend is related to the complex impedance behavior (see Fig. 5 and associated comments). The solvent together with CNT–dA₁₅ present lower dielectric losses, appreciating a clear increment in the conductivity with CNT–dA₁₅–1Pd and even higher with the 1Pd and 1Pd–dA₁₅ solutions, a behavior which is mirrored in the solution's impedance. The observed increase in for CNT–dA₁₅–1Pd compared to CNT–dA₁₅ can be attributed to energy dissipation resulting from the movement of charges in the alternating electric field.⁵³ Therefore, since the dielectric losses shown by CNT–dA₁₅ are indistinguishable from those of the solvent, this increment serves as an indication of the charge transfer process within the multicomponent hybrid.

Conclusions

In this study, we have demonstrated that by exploiting the specific self-assembly properties of the 1Pd–dA₁₅ system, we achieved the formation of continuous one-dimensional Pd(II) arrays directly on DNA strands immobilized on the CNT surface. Circular dichroism spectroscopy confirmed that the Pd–DNA architecture formed on the nanotubes reproduces the same ordered helical arrangement previously observed in solution, indicating that the structural integrity of the Pd nanoarrays is preserved upon integration into the CNT-based hybrid. HRTEM/EDS measurements further confirmed the multicomponent architecture of the CNT–dA₁₅–1Pd hybrids, revealing the presence of P, N, and Pd atoms distributed along the nanotubes, consistent with the localization of both dA₁₅ and the 1Pd complexes on the CNT framework. Importantly, electrical impedance spectroscopy revealed that the assembly of the Pd(II) complexes on the DNA-functionalized CNTs modulates the dielectric response of the hybrid material. The incorporation of the Pd nanoarrays leads to a measurable decrease in impedance and an increase in dielectric loss, consistent with Pd-to-CNT charge transfer mediated by the DNA scaffold.

Overall, this work shows that DNA–Pd architectures can be reliably formed on CNTs and that their nanoscale organization has a direct impact on the electronic behavior of the resulting hybrid. These findings establish a proof of concept for the design of CNT-based materials whose electrical properties can be tuned through the programmed self-assembly of metal complexes along DNA templates.

Experimental

The materials and reagents used in this study include commercial NC3100™ thin multiwalled carbon nanotubes (CNTs) from Nanocyl SA (Belgium), characterized by a high purity of 95+% carbon. The single-stranded DNA oligonucleotide (dA₁₅) from Sigma-Aldrich (MO, USA) consists of fifteen 2′-deoxyadenosine bases with a terminal amino group (–NH₂) at the 3′ position (C₁₅₃H₁₈₉N₇₆O₇₆P₁₅, molecular weight = 4773.26). The formula of the palladium complex (1Pd) is [Pd(Cheld)(CH₃CN)], where Cheld represents chelidamic acid (C₇H₅NO₅). The synthesis of the 1Pd complex, the formation of the 1Pd–dA₁₅ hybrids and the procedures used in this study have been reported elsewhere.³⁶ We also used 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC, C₈H₁₇N₃·HCl) and N-hydroxysuccinimide (NHS, C₄H₅NO₃), both sourced from Sigma-Aldrich (MO, USA) and used as commercial grade and for synthesis, respectively. N,N-Dimethylformamide (DMF, C₃H₇NO), obtained from Sigma-Aldrich (MO, USA), met ACS reagent standards with a purity of ≥99.8%. Additionally, 3-(N-morpholino) propanesulfonic acid (MOPS, C₇H₁₅NO₄S) was acquired from Sigma-Aldrich (MO, USA) in BioUltra grade for molecular biology applications. Potassium permanganate (KMnO₄), also from Sigma-Aldrich (MO, USA), was employed as ACS reagent with purity of ≥99.0%. Milli-Q® water, with a resistance of 18.2 mΩ cm⁻¹ (Type I, ultrapure), was used throughout the experiments.

Grafting dA₁₅ onto carbon nanotubes

The CNTs were oxidized with KMnO₄ to functionalize them with carboxyl groups (–COOH),⁵⁴ resulting in the CNT–COOH sample. See Experimental Details Section in SI. Then, the covalent anchoring reaction of the dA₁₅ oligonucleotide to CNT–COOH was mediated by the prior activation of carboxyl groups with EDC and NHS.^55,56 This coupling reaction ultimately forms amide groups (–CONH–) bridging the nanotube and dA₁₅. The resulting hybrid was labeled as CNT–dA₁₅. In detail, in the first step, CNT–COOH (55 μg) was dispersed in 25 mM MOPS_(aq) buffer with pH = 6.5 (189 μL) via sonication. Then, coupling agents EDC (0.4 M, 11.70 mg) and NHS (0.027 M, 0.58 mg) were added (NHS : EDC molar ratio of 1 : 15) allowing to stand for 15 minutes. In the second step, 311 μL of 100 μM dA₁₅ solution (in 25 mM MOPS_(aq) buffer) were added to the previously activated CNT–COOH dispersion and, subsequently, kept undisturbed at 4 °C, overnight. The resulting suspension was centrifuged (14 000 rpm, 10 min), and the solid washed repeatedly with MOPS buffer until the supernatant did not reveal the presence of dA₁₅ (by measuring UV absorption at 260 nm, Fig. S2). The CNT–dA₁₅ system was characterized using XPS, HRTEM/EDS and FTIR spectroscopy. It is worth mentioning that the sample contained CNT–dA₁₅ of different nanotubes sizes, as commonly observed for commercially available carbon nanotubes.

Self-assembly of complex 1Pd with CNT–dA₁₅

The soluble fraction of CNT–dA₁₅ was isolated in MOPS buffer from the insoluble part. To solubilize it, the CNT–dA₁₅ hybrid solid was immersed in 1 mL of 1 mM MOPS_(aq) buffer (pH = 6.8), sonicated for 30 seconds, and stirred for 2 hours at 50 °C. After separation via centrifugation (14 000 rpm, 10 min), the supernatant was analyzed through UV-vis spectroscopy to confirm the presence of solubilized CNT–dA₁₅ and estimate the concentration of the oligonucleotide (Fig. S1). This allowed calculating the number of adenines bases available to coordinate with 1Pd. Considering the amount required to achieve a stoichiometric 1Pd : adenine ratio of 1 : 1 as one equivalent, a titration was performed by adding controlled amounts of complex 1Pd (incremental equivalents) to monitor the self-assembly. This titration was tracked by circular dichroism (CD). The resulting multicomponent material is named CNT–dA₁₅–1Pd. In detail, the solubilized CNT–dA₁₅ sample (stock solution) for the CD spectroscopy titration series presented an oligonucleotide concentration of ca. 0.8 µM (see Fig. S1). An initial CNT–dA₁₅ solution was elaborated by mixing 350 µL of the stock solution with 350 µL of ultrapure water. To this initial solution, the aliquots from a stock solution of 1Pd (500 µM, DMF) were added, along with comparable aliquots from the CNT–dA₁₅ stock solution to keep the concentration constant. Once the titration experiments have finalized, when 2 equivalents of the complex 1Pd were present in the solution, the resulting solution contained 6% of DMF. An equivalent control experiment was performed using a CNT–COOH dispersion whose concentration was adjusted by UV-Vis normalization to match the optical density of CNT–dA₁₅, excluding the 260 nm band associated with DNA absorption.

Physicochemical characterization

Different techniques were implemented to achieve a comprehensive physicochemical characterization. Ultraviolet-visible (UV-vis) spectroscopy was conducted using a Varian Cary-Eclipse spectrophotometer, equipped with a xenon lamp (peak power 75 kW), a CZERNY-Turner monochromator, and an R-298 red-sensitive photomultiplier tube. Fourier transform infrared (FTIR) spectroscopy was performed on a Jasco FT/IR-460 Plus spectrophotometer, covering the range of 400–4000 cm⁻¹ with a resolution of 4 cm⁻¹ and 64 scans. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Kratos Axis Ultra-DLD spectrometer with an Al/Mg twin anode. Monochromatic Kα radiation was used in constant-analyzer-energy mode with pass energies of 160 and 20 eV for survey and high-resolution spectra, respectively. High-resolution transmission electron microscopy (HRTEM) coupled with energy dispersive X-ray spectroscopy (EDS) was performed using an FEI TITAN G2 microscope with selectable acceleration voltage from 50 to 300 kV, a maximum magnification of 1.25 MX, a resolution of 0.8 Å, and an XFEG-type field emission gun with a Schottky-type filament. The EDS analysis was conducted using the FEI microanalysis system equipped with four detectors and an image drift correction system. Finally, circular dichroism (CD) spectroscopy was carried out with a Jasco J-815 spectropolarimeter, which includes a Peltier temperature controller.

Measurements of dielectric properties

EIS measurement setup details & calibration. In the pursuit of characterizing the electrical properties of various solutions making use of the Electrochemical Impedance Spectroscopy (EIS) technique, the Keysight E4990A impedance analyzer has been employed, operating within a frequency range spanning from 20 Hz to 120 MHz.⁵⁷ The analyzer (Fig. S13) was supplemented with the Keysight 16048G port extension⁵⁸ in conjunction with the Keysight 16452A Liquid Test Fixture,⁵² designed for experimental measurements within the frequency range of 20 Hz–30 MHz. The adoption of a 4-terminal pair (4TP) measurement technique serves to minimize measurement errors by mitigating the influence of parasitics such as contact resistances, residual impedances, and short and open offsets. The calibration process entails a sequential methodology beginning with the calibration of the port extension (16048G) employing a 100 Ω load (Keysight E4990-61051). Then, the 16452A fixture was calibrated incorporating a short compensation procedure. Stringent criteria are applied to ensure calibration accuracy, requiring that the residual series inductance remains below 20 nH, and the series resistance is maintained below 0.5 Ω.⁵²

Dielectric properties extraction. The extraction of the dielectric properties of the solution encompasses two distinct measurements: first, a reference measurement conducted with air-filled fixture, serving as the dielectric medium between two parallel plates, enables the determination of the reference capacitance (C₀) across the desired frequency range. Second, the fixture is filled with the solution and the measurement is carried out in the same frequency range. The auxiliary reference measurement is instrumental in rectifying or mitigating potential measurement errors arising from minor fluctuations in plate separation distance and electrode effective area. The parallel plate spacing was 3 mm, with a required liquid volume of 6.8 ml. The air reference yielded C₀ = 5.3 pF at 1 MHz, close to the target value of 5.5 ± 10% pF at the same frequency.

By employing the measured parallel plate capacitance (C_m) and resistance (R_m), in conjunction with C₀, the complex dielectric constant () is evaluated as:

where ω is the angular frequency. To account for fringing effects of the 16452A fixture, a correction factor α was applied:where ε_r is the corrected complex dielectric constant. Furthermore, the conductivity (σ) is determined by the dielectric losses ():with ε₀ the vacuum permittivity.

The accurate extraction of the dielectric properties requires that the equivalent model of the measured impedance conforms to a parallel R|C configuration. The analysis of the Nyquist plot indicates that for frequencies above 5 kHz, this equivalent model of the Samples Under Test (SUT) can be assumed. Nevertheless, in instances involving samples with elevated conductivity, such as cell suspensions, the polarization impedance at the electrodes, operating in series with the SUT impedance, cannot be disregarded. Various methodologies exist to approximate this behavior, typically involving attempts to offset its influence.^44,45 In the context of the current investigation, attention is directed towards a frequency range wherein this effect can be deemed negligible.

Although the operational frequency range of the experimental setup (E4990A impedance analyzer, 16048G extension port, and 16452A fixture) covers the 20 Hz–30 MHz range, we limit the usable window to 1 kHz–5 MHz to minimize noise and polarization artifacts. At sufficiently low frequencies (<1 kHz), the polarization impedance Z_pol (modelling the electrode behavior) becomes dominant, as evidenced in Fig. S14a and based on the Nyquist plot, for frequencies below 5 kHz, the SUT impedance no longer fits a parallel RC model. Polarization impedance typically exhibits a reactive nature (as the compounds under study), converging towards the behavior of the solution as the excitation frequency increases. Such behavior does not manifest in the conductivity of the composites, as depicted in Fig. S14b, given that conductivity is directly associated with the real part of the measured impedance, related to R_m.

Conflicts of interest

There are no conflicts to declare.

Author contributions

V. K. A.-F. and A. P.-R. prepared and characterized the molecular systems and contributed to data analysis and discussion. M. G.-P., F. G. R., and A. G. performed the electronic studies, and analyzed and discussed the corresponding data. M. A. G. and V. K. A.-F. conceived the research and wrote the original draft, leading manuscript preparation. M. A. G. and M. P.-M. supervised the project, analyzed the data, and secured funding. All authors reviewed and approved the final version of the manuscript.

Data availability

All data supporting the findings of this study, including experimental results and characterization data, are provided in the supplementary information (SI). Supplementary information: experimental details: oxidation of CNT via KMnO₄ treatment. Figures: S1, UV-vis spectrum of the solubilized CNT–dA₁₅ dispersion; S2, UV-vis spectra of CNT–COOH and dA₁₅ reaction medium at initial and final times, and the supernatants in wash repetitions; S3, XPS wide spectra of the CNT, CNT–COOH and CNT–dA₁₅ samples; S4, XPS high-resolution C 1s deconvolutions of the CNT, CNT–COOH and CNT–dA₁₅ samples; S5–S6, HRTEM images, EDS spectra and element distribution maps of the CNT–dA₁₅ hybrid; S7, FTIR spectrum of the CNT–dA₁₅ hybrid; S8–S10, HRTEM images, EDS spectra and element distribution maps of the multicomponent CNT–dA₁₅–1Pd hybrid; S11, (a) parallel capacitance–resistance circuit corresponding to the equivalent electrical model of the SUT, (b) parallel capacitance–resistance circuit including Warburg impedance (Zω); S12, Fitted parameters for the R_pC_p model; S13, Illustration of the EIS measurement setup; S14, (a) relative dielectric constant (ε′r) and (b) conductivity in the frequency range 1 kHz–5 MHz. See DOI: https://doi.org/10.1039/d5qi02340h.

Acknowledgements

Financial support was received from Grant PID2020-120186RB-I00 and PID2023-152026NB-I00 funded by MCIN/AEI/10.13039/501100011033, Grant P20_00702 funded by Consejería de Universidad, Investigación e Innovación and “FEDER – a way of making Europe”, Junta de Andalucia. V. K. A.-F thanks the ElectroPlas project (EMERGIA program, EMEC_2023_00163, Plan Andaluz, D. 54/1989, Consejería de Universidad, Investigación e Innovación, Junta de Andalucía) for its financial support. M. G.-P. acknowledges the grant PREP2023-001936 funded by MICIU/AEI/10.13039/501100011033 and FSE+.

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Footnote

† These authors contributed equally to this work.

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