AuNP based selective colorimetric sensor for cysteine at a wide pH range: investigation of capping molecule structure on the colorimetric sensing and catalytic properties

Abstract

Gold nanoparticles (AuNPs) stabilized with different surfactants, SDS, PEG, PVA, PVP, PSS and T-80, were synthesized and explored for cysteine colorimetric sensing and catalytic properties. The sensing and catalysis studies revealed an interesting observation that AuNP surfaces covered with linear molecules, SDS and PEG, did not show any colorimetric sensing and exhibited the lowest catalytic effect. Whereas moderately improved sensing and catalysis was observed with AuNPs covered with smaller functional group-substituted PVA and PVP. Interestingly, a selective and robust colour change for cysteine (10⁻⁷ M) in aqueous solution as well as the strongest catalytic effects were observed with AuNPs covered with bulky functional group-substituted PSS and highly branched T-80. These differences could be attributed to the surface accessibility of the AuNPs for the analytes. HR-TEM analysis of T-80–AuNPs with cysteine clearly showed the formation of smaller aggregates of AuNPs. Importantly, T-80–AuNPs showed selective sensing of cysteine across a wide pH range (2.0–10.0). PSS–AuNPs showed selective colorimetric sensing only in the pH range of 6.0 to 10. These studies suggest that the surface capping molecule structure plays a very significant role in both colorimetric sensing and catalysis by AuNPs.

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Introduction

In recent years, there has been a strong interest in exploiting the unique optical and catalytic properties of AuNPs in order to develop efficient colorimetric sensors and catalysts as well as gaining more insight on the mechanism.^1,2 Colorimetric sensors offer several advantages such as their simplicity, cost effectiveness and ability to allow on-site monitoring of analytes.^3–5 AuNP based colorimetric sensors have received significant attention because of their strong surface plasmon resonance and distance dependent optical properties.^6,7 Furthermore, AuNPs posses stronger extinction coefficients compared to organic dyes, which makes them very suitable for colorimetric sensing systems.^8,9 The interaction of the NPs with analytes induces a rapid visible color change due to the coupling of the interparticle surface plasmon,^10–14 that provides a practical platform for the colorimetric detection.

Cysteine is an essential amino acid for the human body and can be found as a component of many proteins. Cysteine plays a crucial role in a variety of important cellular functions including protein folding, detoxification, metabolism and redox processes.^15,16 It also acts as a physiological regulator in various diseases such as heart disease, rheumatoid arthritis, and AIDS.^17,18 Cysteine deficiency can cause many syndromes, such as hair depigmentation, oedema, liver damage, skin lesions, lethargy and loss of muscle and fat.^19,20 Thus, the development of suitable approaches for the selective detection of cysteine in various samples has attracted considerable interest in recent years. A variety of detection techniques including high performance liquid chromatography,²¹ chemiluminescence,²² electrochemistry,²³ optical spectroscopy,²⁴ and capillary zone electrophoresis²⁵ have been developed for the determination of cysteine. However, most of these techniques are complicated, require expensive instrumentation and are not suitable for routine analysis. Noble metal nanoparticles such as Ag and Au are employed for sensing sulfur-containing amino acids by utilizing their thiophilic nature.^26,27 For example, oligonucleotide-functionalized AuNP assays were developed for the highly sensitive and selective colorimetric detection of cysteine.²⁸ Water soluble quaternized cellulose based AuNPs were recently fabricated for the selective sensing of cysteine.²⁹ Citrate stabilized AuNPs were recently used for the selective sensing of cysteine caused by the self-assembly of NPs by supramolecular interactions with aspartic acid.³⁰ In most cases, AuNPs are modified with oligonucleotide- or thiol-containing organic molecules to enable color or light intensity changes. These assays have some advantages, but they are still cost-consuming and require targeted structural modification. And also cysteine sensing at wide pH range has never been explored.

The application of AuNPs in catalysis is another exciting area of research.^2,31 AuNPs immobilized on polymer and solid matrixes were synthesized and studied for their catalytic activities.³² In particular, redox reactions such as CO oxidation³³ and the hydrogenation of various compounds³⁴ were used to test the catalytic capability of the AuNPs. AuNPs with well defined size and shape were synthesized and the catalytic properties were explored to gain more insight into the catalytic performances. Smaller particle sizes showed increased catalytic activities.³⁵ Fenger et al. reported the highest activity for 13 nm sized AuNPs.³⁶ AuNPs with spherical morphology showed stronger catalytic activities than prismatic and nanorod morphologies.³⁷ Herein, we report the synthesis of AuNPs stabilized with six different capping ligands, SDS, PEG, PVA, PVP, T-80 and PSS and investigate the cysteine amino acid colorimetric sensing and catalytic properties. AuNPs stabilized with linear capping molecules such as SDS and PEG exhibited the lowest catalytic effects and no colorimetric sensing. Capping ligands with small branched functionality such as PVA and PVP showed slightly improved catalytic effects and colorimetric sensing. In contrast, T-80 and PSS, which have bulky side group functionality, showed robust sensing of cysteine and the fastest rate of reduction of 4-nitrophenol. Importantly, the T-80–AuNPs showed selective colorimetric sensing of cysteine across a wide pH range (2.0–10.0) whereas the PSS–AuNPs exhibited selective sensing in the pH range of 6.0 to 10.0.

Materials and methods

Sodium dodecyl sulfate (SDS), poly(ethylene glycol) (PEG), poly(vinyl alcohol) (PVA), poly(vinyl pyrrolidone) (PVP), sodium salt of poly(styrene sulfonate) (PSS), polysorbate-80 (T-80), HAuCl₄, ascorbic acid, 4-nitrophenol and amino acids were obtained from Sigma-Aldrich and used as-received. Milli-Q water was used for the preparation of the AuNPs with all stabilizing agents. All of the amino acid solutions used for the experiments were prepared by mixing the requisite amount of amino acid in Milli-Q water.

General preparation of SDS, PEG, PVA, PVP, PSS and T-80–AuNPs

10 ml of aqueous solution containing 0.1 mM HAuCl₄ and 1.0 mM of SDS (0.5 wt% in the case of the other polymeric capping agents) was placed in a 25 ml beaker and stirred at 60–70 °C. The addition of 1 ml of 1.0 mM ascorbic acid immediately turned the yellow solution into a wine-red colloid dispersion. The solution was allowed to stir at 60–70 °C for another 30 min. The reactions were repeated at least three times to confirm the reproducibility of the AuNPs formation. The characterization of the synthesized AuNPs was carried out after allowing the solution to stand at room temperature for more than one week.

Characterization

The UV-visible measurements of the AuNPs stabilized with six different ligands were performed in a Perkin Elmer model UV-Vis double beam spectrophotometer from 250 to 800 nm, at a resolution of 1 nm. The purified powders of the PSS–AuNPs and cysteine-added AuNPs were subjected to FT-IR spectroscopy measurements. These measurements were carried out on a Perkin-Elmer Spectrum-One instrument in the diffuse reflectance mode at a resolution of 4 cm⁻¹ in KBr pellets.

The size and morphology of the AuNPs were investigated using high resolution transmission electron microscopy (HR-TEM). Samples for TEM measurements were prepared by placing a drop of NP solution on a graphite grid and drying it under vacuum. Transmission electron micrographs were taken using a JEOL JEM-2100F instrument operating at an accelerated voltage of 200 kV and using an ultra high-resolution pole piece.

Zeta potential measurements were carried out using a Zetasizer ver.6.20 instrument. An aqueous suspension of the silver nanoparticles was placed in a cuvette. A zeta potential was measured by the principle of electrophoretic mobility created by applying an electric field across the dispersion media.

Catalytic reaction

A standard catalytic test reaction was carried out in a 2.0 ml quartz cuvette. 2 ml of an aqueous solution of 0.1 mM p-nitrophenol was mixed with 1 ml of a 0.1 M NaBH₄ solution. The reaction was started by the addition of a 50 μl sample of the AuNPs prepared using the above-mentioned procedure. Only 10 μl of the PSS–AuNP solution was added in the case of the nitrobenzene reduction, since the addition of 50 μl completed the reduction immediately. The reaction was vigorously magnetically stirred to avoid diffusion limitation. Immediately after catalyst addition, time dependent absorption spectra were collected at 3 min intervals for at least 30 min at room temperature. The background subtraction was performed with deionized water as the reference. All spectra were corrected with an average value at around 800 nm due to interferences during the catalytic reaction caused by the evolving gas bubbles (hydrogen release).

Result and discussion

Addition of ascorbic acid to an aqueous solution of Au³⁺ in the presence of a stabilizing agent resulted in the formation of stable AuNPs with a wine-red colour. The clear and transparent wine-red colour, which was due to the strong surface plasmon resonance (SPR) vibration,³⁸ confirmed the good dispersion of the AuNPs. The absorption studies of the AuNPs capped with different ligands revealed typical SPR absorption in the range of 520 to 530 nm (Fig. 1). The size, morphology and crystallinity of the samples were analyzed using HR-TEM, which clearly showed the formation of spherical crystalline polydispersed AuNPs in the size range of 3 to 15 nm (Fig. 2). Various morphologies, including spheres and triangular prisms, were observed in the SDS–AuNPs (Fig. 2a and S1†). From the zeta potential measurements, it can be confirmed that both SDS and PSS stabilize AuNPs better than other ligands (Table S1†). All other capping molecules (PEG, PVA, PVP, T-80) exhibited similar AuNP stabilization.

Fig. 1 (a) Structures of capping ligands and (b) absorption spectra of AuNPs synthesized using different capping ligands.

Fig. 2 HR-TEM images of (a) SDS–AuNPs, (b) PEG–AuNPs, (c) PVA–AuNPs, (d) PVP–AuNPs, (e) PSS–AuNPs and (f) T-80–AuNPs.

AuNPs are known to form strong interactions with thiol groups due to their thiophilic nature.^26,27 This property has been utilized for the selective colorimetric sensing of thiol-functionalized amino acids which undergo a colour change from wine-red to blue. The synthesized AuNPs were also investigated for their colorimetric sensing of different amino acids, (glycine (Gly), alanine (Ala), serine (Ser), valine (Val), leucine (Leu), phenylalanine (Phe), tryptophan (Trp), histidine (His), cysteine (Cys), methionine (Met), tyrosine (Tyr) and glutathione (GSH)) in aqueous solution by monitoring the colour and absorption change. It should be noted that Cys, Met and smallest tripeptide GSH all possess thiol functionality. SDS– and PEG–AuNPs did not show any significant change in absorption or colour with the addition of different amino acids even at higher concentrations (10⁻² M), except for a small decrease in absorption intensity without altering λ_max (Fig. S2†). However, PVA–, PVP–, T-80 and PSS–AuNPs showed selective colorimetric changes from wine-red to blue upon the addition of the cysteine amino acid (Fig. 3 and 4). The addition of other amino acids, including methionine and glutathione, did not result in any colour change. The absorption studies of PVA– and PVP–AuNPs with cysteine have also confirmed the red shifting of λ_max (Fig. 3). The minimum detectable concentration of cysteine was determined by adding different volumes of a 10⁻⁶ M solution of the amino acid into solutions of the PVA– and PVP–AuNPs (Fig. S3†). The AuNP absorption λ_max was completely red shifted by the addition of 280 μl (for PVA–AgNPs) and 340 μl (for PVP–AgNPs). It was noted that the AuNP absorption λ_max was red shifted by 6 and 18 nm upon addition of cysteine to PVA– and PVP–AuNPs, respectively.

Fig. 3 Cysteine colorimetric sensing studies of (a) PVA–AuNPs and (b) PVP–AuNPs. Digital images of the colour change are shown in the inset.

Fig. 4 Cysteine colorimetric sensing studies of (a) PSS–AuNPs and (b) T-80–AuNPs. Digital images of the colour change are shown in the inset.

Interestingly, the PSS– and T-80–AuNPs showed strong red shifting of the absorption λ_max with robust sensing of cysteine compared to the PVA– and PVP–AuNPs. In particular, the PSS–AuNPs exhibited the strongest absorption red shift upon addition of cysteine (535 to 610 nm). The T-80–AuNP absorption red shifted from 525 to 560 nm upon addition of cysteine. Concentration dependence studies of T-80–AuNPs with cysteine showed a clear absorption red shift even after the addition of 80 μl (10⁻⁷ M) which was completed by the addition of 120 μl (Fig. 5). Similarly, the absorption λ_max of the PSS–AuNPs was red shifted from 535 nm to 610 nm after the addition of 60 μl. The red shifting was completed after the addition of 80 μl of cysteine. Further additions of cysteine did not show any significant shift in the λ_max, rather, it only reduced the absorption intensity. The reduction of the absorption intensity was due to the formation and settling down of smaller AuNP aggregates with cysteine. The effect of the presence of other amino acids on the selective colorimetric sensing of cysteine by PSS– and T-80–AuNPs was also studied (Fig. S4†). The absorbance change of the AuNPs toward cysteine in presence of other amino acids, including methionine and glutathione, do not interfere with the cysteine binding with AuNP probes or subsequent changes in absorption, indicating that the presence of other amino acids had a negligible interfering effect on cysteine sensing.

Fig. 5 Cysteine concentration dependence studies of (a and c) PSS–AuNPs and (b and d) T-80–AuNPs.

The selective sensing of cysteine by T-80– and PSS–AuNPs was also explored at different pHs (Fig. 6). The pH of the T-80– and PSS–AuNP solutions was tuned by adding dilute HNO₃/NaOH solution. Absorption studies of both T-80– and PSS–AuNPs at different pHs did not show any significant variation and confirms the good stability of the AuNPs (Fig. S5†). Interestingly, AuNPs stabilized with a nonionic ligand, T-80, exhibited selective colorimetric sensing of cysteine across a wide pH range from 2.0 to 10.0. But the PSS–AuNPs showed cysteine sensing only in the pH range of 6.0 to 10.0. It should be noted that the T-80–AuNPs exhibited the strongest red shifting of the absorption λ_max with cysteine under acidic conditions, whereas PSS–AuNPs showed red shifting under basic conditions.

Fig. 6 pH dependent cysteine colorimetric sensing of (a) PSS–AuNPs and (b) T-80–AuNPs. The insets show digital images of the cysteine sensing.

The mechanism of the selective sensing of cysteine by AuNPs is believed to be due to the thiophilic interaction of sulfur that leads to the formation of AuNPs aggregates. The formation of aggregates changes the colour of the AuNPs from wine-red to blue. HR-TEM studies of T-80–AuNPs with cysteine clearly demonstrate the formation of AuNP aggregates (Fig. S6a†). PVP–AuNPs with cysteine also exhibited the formation of AuNP aggregates but to a lesser extent than the T-80–AuNPs (Fig. S6b†). However, SDS–AuNPs with cysteine did not show any significant aggregation and supports the observation of no colour or spectral changes (Fig. S6c†). Theoretical and experimental studies have shown that the plasmon oscillations of metal nanoparticles couple to each other when they are brought into proximity and exhibit different colours.^39,40 The comparison of selective cysteine colorimetric sensing of AuNPs with six different capping ligands, SDS, PEG, PVA, PVP, T-80 and PSS revealed a very interesting trend. The linearly structured SDS and PEG capped AuNPs did not show any colorimetric sensing. However, substitution of the bulky functional groups in the capping ligands, with T-80 and PSS showed robust colorimetric sensing of cysteine. AuNPs with PVA and PVP in which smaller functional groups are attached showed moderate colorimetric sensing and catalytic properties. Thus, the difference in the colorimetric sensing of AuNPs with SDS, PEG, PVA, PVP, PSS and T-80 might be attributed to the structural and shape differences of the capping molecules. The diffusion of cysteine molecules onto the NP surface could be a critically important step for colorimetric sensing. It should be noted that, with the exception of SDS, all samples showed polydispersed spherical sized AuNPs. Hence the role of size or shape on the colorimetric sensing differences can be excluded. Zeta potential measurements showed that PSS stabilizes AuNPs better than PEG, yet exhibited robust sensing of cysteine and excluded the effect of stabilization from the different capping molecules (Table S1†). The above results indicate that linear capping molecules (SDS and PEG) provide better packing around AuNPs and hence cysteine molecules might not be able reach the AuNP surface. Whereas, the substitution of bulky functionality (PSS, T-80) that could form more void packing due to steric hindrance provided easier access for cysteine to the surface of the AuNPs, and exhibited robust sensing (Scheme 1). Furthermore, to confirm the surface accessibility differences, the catalytic performance of AuNPs stabilized with six different capping ligands was also studied.

Scheme 1 Schematic representation of SDS and PSS packing around the AuNPs.

Nitro group reduction by NaBH₄ in presence of AuNPs has industrial relevance in the preparation of aniline and paracetamol, and is a standard reaction to test the catalytic capability of nanoparticles.⁴¹ The conversion of 4-nitrophenol to 4-aminophenol and nitrobenzene to aniline by AuNPs were explored using a standard UV-Vis setup. A strong yellow coloured 4-nitrophenolate was formed by the addition of NaBH₄ to 4-nitrophenol. The absorption spectrum showed a strong band (λ_max) at 400 nm. Time dependent absorption spectra were recorded in order to monitor the reaction progress after adding AuNPs to the reaction solution. The solution was stirred after the addition of the AuNPs to ensure an immediate uniform distribution. The intensity of the absorption correlates with the concentration of 4-nitrophenolate. Fig. 7 shows the time-dependent evolution of the absorption spectra for the reaction catalysed by PSS–AuNPs. The intensity of the 4-nitrophenolate absorption band at 400 nm disappeared completely within 30 min, indicating the successful reduction of 4-nitrophenol to 4-aminophenol. Similarly nitrobenzene was reduced to aniline within 30 min by PSS–AuNPs. The appearance of a new peak at 230 nm was taken as an indication of aniline formation. Furthermore, to confirm the catalytic activity of the AuNPs, the same reaction was performed in the absence of PSS–AuNPs. This solution remained unchanged even for a week, thus indicating that 4-nitrophenol was not reduced without the catalyst. Since NaBH₄ was present in excess (1000 : 1) with respect to 4-nitrophenol, this reaction can be handled under pseudo-first order conditions.

Fig. 7 Catalytic reduction studies using PSS–AuNPs for (a) 4-nitrophenol and (b) nitrobenzene.

The structural differences between the AuNP capping ligands provided an opportunity to study the structure dependent catalytic activity of the AuNPs. It was noted that AuNPs with six different capping ligands exhibited different cysteine colorimetric sensing. A comparison of the catalysis of 4-nitrophenol to 4-aminophenol and nitrobenzene to aniline by AuNPs stabilized with six different ligands is shown in Fig. 8. AuNPs stabilized with SDS and PEG capping ligands exhibited the lowest or almost negligible catalytic activity for the reduction of 4-nitrophenol and nitrobenzene. PVA and PVP stabilized AuNPs exhibited higher activities than SDS and PEG–AuNPs. However, T-80 and PSS stabilized AuNPs showed the strongest catalytic activity in both reactions. Nitrobenzene was reduced to aniline within 15 min and further reaction did not show any absorption change at 230 nm. Whereas the increased absorption at 230 nm with time for the PVA– and PVP–AuNPs suggested incomplete reduction even after 30 min. Similarly, T-80– and PSS–AuNPs almost completely reduced 4-nitrophenol to 4-aminophenol within 30 min. The PVA–AuNPs showed better activity than the PVP–AuNPs and similar activity to the T-80 and PSS–AuNPs in the 4-nitrophenol reduction. Based on the catalytic activities, the AuNPs capped with six different ligands were grouped into three categories: the least active SDS– and PEG–AuNPs, moderately active PVA– and PVP–AuNPs and most active T-80– and PSS–AuNPs. The selective cysteine colorimetric sensing also showed a similar trend; no sensing by the SDS– and PEG–AuNPs, moderate sensing by the PVA– and PVP–AuNPs and robust sensing by the T-80– and PSS–AuNPs.

Fig. 8 Comparison of the catalytic reduction of (a) 4-nitrophenol and (b) nitrobenzene with AuNPs capped with different ligands.

The structural comparison of the six different capping ligands showed an interesting trend, from linear to bulky functional group substitution on the side chain. For colorimetric sensing as well as catalysis, the efficient interaction of the analytes/reactant with the AuNP surface is very important and hence any hindrance is expected to reduce both the sensing performance as well as the catalytic activities. We have assumed that both the colorimetric and catalytic differences between the AuNPs might be due to the surface packing differences between the capping ligands (Scheme 1). It appears that linearly structured SDS and PEG pack more densely, thus completely blocking the access of the analyte/reactant to the AuNP surface. The dense coverage of the AuNP surface by the lysozyme protein showed the lowest catalytic activity.⁴² The smaller functional group substitution in PVA and PVP might provide restricted access for the analytes/reactants to the AuNPs surface in PVA and PVP capped AuNPs. However, T-80 and PSS with bulky functional group substitution might cover the AuNP surface with a more porous structure, thus providing easier access for the analyte/reactant to the NPs surface and hence strongest activity in both sensing and catalysis. These results clearly indicate that packing of the capping ligands on the surface of the AuNPs and their structures could control both their colorimetric and catalytic properties.

Conclusion

In summary, capping ligands that are known to have a strong influence on the optical properties of noble metal NPs, have played an important role in colorimetric sensing and catalysis. AuNPs stabilized with linear molecular structures (SDS and PEG) displayed the lowest catalytic activities and no cysteine colorimetric sensing. AuNPs stabilized with small functional group substituted ligands (PVA and PVP) exhibited moderate catalytic activities and sensing properties. Interestingly, a selective and robust colour change for cysteine in aqueous solution and the strongest catalytic activity were observed with AuNPs stabilized with bulky functional group substituted PSS and highly branched T-80. The formation of polydisperse spherical AuNPs for all samples except SDS rule out the effects of size and shape on the colorimetric and catalytic differences. Similarly, zeta potential measurements exclude the stabilization effect since better stabilized PSS–AuNPs showed stronger colorimetric and catalytic activities compared to the PEG–AuNPs. HR-TEM analysis of the T-80–AuNPs with cysteine clearly showed the formation of smaller aggregates of AuNPs. Furthermore, pH dependent colorimetric sensing studies of T-80–AuNPs showed selective sensing of cysteine across a wide pH range (2.0–10.0). PSS–AuNPs showed selective colorimetric sensing only in the pH range of 6.0 to 10. We believe that the present studies could be useful in choosing noble metal NP surface capping ligands for the development of efficient colorimetric sensors and catalysts.

Acknowledgements

Financial support from the Department of Science and Technology, New Delhi, India (DST Fast Track Scheme no. SR/FT/CS-03/2011(G)) and CRF facility, SASTRA University are acknowledged with gratitude.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Cysteine colorimetric studies of SDS– and PEG–AuNPs, cysteine concentration dependent studies of PVA– and PVP–AuNPs, PSS–, T-80–AuNPs absorption at different pH and formation of smaller AuNPs aggregation due to selective cysteine interaction with PSS–AuNPs. See DOI: 10.1039/c4ra00345d

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