Fragmentation of supported gold nanoparticles@agarose film by thiols and the role of their synergy in efficient catalysis

Abstract

Thiols are known for stabilizing gold nanoparticles (Au NPs). Herein, an intriguing feasibility of a group of thiols to non-conventionally fragment supported Au NPs in the absence of any auxiliary applied energy is demonstrated. The Au NPs are studded into an agarose hydrogel film as a solid support (Au@Agr), and the group of thiols studied includes thioglycolic acid (TGA), cysteine (CS), 2-mercaptoethanol (ME), L-methionine (MET), thiourea (TU), and 1,3-propanedithiol (PDT). By simply keeping the Au@Agr film immersed into thiol solution for 1 h at room temperature, fragmented nanoparticles of much smaller sizes were obtained. Interestingly, only TGA, CS, ME and MET were found to be capable of successfully fragmenting Au@Agr. A mechanistic interpretation suggests that a prompt transfer of electron density from thiols to Au@Agr can be credited for the fragmentation behaviour of thiols. In addition, the efficacy of such a film in showing an efficient catalytic activity regulated by synergism between thiols and supported Au NPs (Au@Agr) is also presented. TGA and CS fragmented Au@Agr, i.e., TGA-Au@Agr and CS-Au@Agr films, work as an effective catalyst taking ∼20 to 30 seconds for the complete reduction of p-nitrophenol (p-NP), an industrial pollutant with sluggish removal properties. A pseudo first-order rate for the catalytic p-NP reduction reaction is followed by TGA-Au@Agr as well as CS-Au@Agr with rate constant values determined to be 1.6 × 10⁻¹ s⁻¹ and 1.1 × 10⁻¹ s⁻¹, respectively. The exclusivity of TGA and CS to swiftly catalyze p-NP reduction and also the individual role of Agr, Au@Agr and TGA/CS-Au@Agr operating in synergy for the successful catalysis was also studied in detail.

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Introduction

The fate and pre-conceptualised nature of gold nanoparticles (Au NPs) took a new turn after the report of thiol-derivatised gold nanoparticles by Brust et al.¹ in 1994. They introduced a two-phase liquid–liquid system for the preparation of Au NPs attached with self-assembled thiol monolayers behaving unusually as a simple chemical compound with moderate polydispersity. Consequentially, thiol-capped Au NPs have been extensively studied over the two decades reporting a various number of papers establishing the role of thiols in stabilizing as well as regulating the size of Au NPs in a solution.^2–9

Due to persistence of aggregation^10–13 related problems, scientists next reported a photophysical fragmentation of these thiol-capped Au NPs induced by a laser.¹⁰ It is believed that this non-conventional perspective of Au NPs fragmentation by thiols is due to the photo-ejection of electrons, which is followed by charging of the nanoparticle surface. Novo et al.¹⁴ in a report showed charge induced fragmentation of gold nanorods by sodium borohydride. As a follow-up, Wang et al.¹⁵ showed fragmentation of Au NPs by the cysteine biomolecule. They demonstrated that upon mixing gold chloride with cysteine and excess of sodium borohydride, Au NPs of 3–5 nm size are first obtained, which are then converted into ultrasmall nanoparticles sized <1.5 nm after 6 h of aging at room temperature. In other reports, Riaz et al. demonstrated thiol-ubiquinone induced fragmentation¹⁶ and thioanisole induced size-dependent fragmentation of Au NPs.¹⁷ The role of alkanethiol fragmented Au NPs in controlling the fluorescence properties of Au NPs for the sensitive and selective detection of Hg(II) was shown by Huang et al.,¹⁸ and thiol induced core etching of lysozyme type VI (Lys VI)-stabilized luminescent Au₈ clusters was shown by Ke et al.¹⁹ However, the feasibility of such non-conventional thiol-mediated fragmentation of Au NPs when supported on a solid platform is still an unexplored area of research.

In this study, we investigated the feasible role of thiols in fragmenting Au NPs supported on an agarose (Agr) hydrogel film. Addressing within the limits of our knowledge, it is the first report to show the non-conventional thiol-mediated fragmentation of Au NPs studded into an Agr hydrogel film solid support as a hybrid material. The Au NPs, unlike other reported methods,^20–23 are generated inside the agarose hydrogel film through adsorption of 5 mM tetrachloroauric salt (HAuCl₄) solution by a preformed Agr hydrogel film, which is subsequently reduced by only 10 min exposure to 365 and 254 nm wavelengths UV radiation. The role of a agarose hydrogel as a solid support in facile and quick generation of Au NPs is also considered here. This hybrid Au@Agr film is then investigated to study the feasibility of such a system to undergo the recently popularized non-conventional fragmentation of Au NPs by a group of thiols (viz., thioglycolic acid (TGA), cysteine (CS), 2-mercaptoethanol (ME), L-methionine (MET), thiourea (TU), and 1,3-propanedithiol (PDT)). The surface plasmon resonance (SPR) peak of Au NPs detected by UV-visible spectrometry strongly depends on its particle size, and any change in the size of Au NPs is reflected by a shift in the SPR peak wavelength either towards lower (blue shift) or higher (red shift) wavelength values.^24,25 Accordingly, to study the fragmentation of Au@Agr by thiols such a blue/red shift in the SPR peak of Au NPs is closely monitored by UV-visible spectrometry. Among all the six thiols, TGA, CS, ME and MET were found to be capable of showing fragmentation of Au@Agr displaying a considerable 6 to 11 nm blue shift in the SPR peak of Au@Agr. Other thiols of the group, i.e., TU and PDT, do not show any sign of fragmentation, instead they have been found to cause red shifts in the SPR peak of Au NPs.

The study also demonstrates application of these films in rapid catalysis of an industrially important reduction reaction of p-nitrophenol (p-NP) to p-aminophenol (p-AP). p-NP is a known pollutant that is very noxious and sluggish, while p-AP is of great importance serving as an intermediate for synthesis of many analgesic, antipyretic drugs, and polymers.^26,27 Moreover, p-NP reduction to p-AP is one of the most ideal reactions studied for establishing catalytic efficiency of a catalyst due to its easy mode of monitoring with UV-visible spectroscopy and occurrence of no side reactions.^26–37 The films (especially, TGA-Au@Agr & CS-Au@Agr) are successfully used as catalysts in the reduction of p-NP to p-AP in the presence of excess sodium borohydride (NaBH₄). The calculated rate constant was found to be 1.6 × 10⁻¹ s⁻¹ and 1.1 × 10⁻¹ s⁻¹ for TGA-Au@Agr and CS-Au@Agr catalysed reduction reactions, respectively, following pseudo first-order kinetics. To the best of our knowledge, this rate constant value of the order 10⁻¹ s⁻¹ is one of the fastest catalytic rates observed for any solid supported catalyst. A thriving synergism between TGA or CS and Au@Agr stimulating a smooth relay of electrons from borohydride ions (BH₄⁻) to p-NP is credited for such a rapid catalysis shown by the TGA-Au@Agr and CS-Au@Agr films. Hence, this report is an endeavour to investigate and establish an intriguing feasibility for non-conventional behaviour of a group of thiols to fragment Au NPs supported in a solid Agr hydrogel film in absence of any auxiliary applied energy. The study also explores the synergy functioning inside the film to make it an efficient catalytic material for reduction of p-NP to p-AP.

Experimental

Materials

Agarose low gelling temperature (SRL, India), tetrachloroauric acid (Sigma Aldrich), thioglycolic acid (Sigma Aldrich), L-cysteine hydrochloride monohydrate (Merck), L-methionine (Sigma Aldrich), 2-mercaptoethanol (Himedia), thiourea (Merck), 1,3-propanedithiol (Sigma Aldrich), p-nitrophenol (Lobal Chemie), and sodium borohydride (Merck) were used as received. All other reagents used were of analytical grade.

Methodical step-by-step protocol to investigate thiol-mediated fragmentation of gold nanoparticles (Au NPs) studded into agarose hydrogel film

Agarose hydrogel film (Agr). Agarose hydrogel of 2.5% (w/v) was prepared following the conventional method of mixing 0.25 g agarose in 10 mL of Milli Q water and allowing microwave dissolution for 25 s. The film casting of the microwaved solution was done by pouring it in a Petri dish (10 mm diameter) and cooling to room temperature, which was then kept for 4 hours (at room temperature) to obtain a thin Agr hydrogel film.³⁸

Au NPs studded agarose hydrogel film (Au@Agr). To the Petri dish containing the prepared Agr hydrogel film, 10 mL of 5 mM HAuCl₄ solution was poured and kept as such for 1 h 15 min. After this, the HAuCl₄ solution was poured out from the Petri dish, and the film was washed thoroughly with plenty of Milli Q water to remove any unreacted HAuCl₄ if present. Then, the HAuCl₄ treated Agr film was exposed to 365 nm and 254 nm wavelength UV light for 10 min. The ruby red coloration of the Agr film after UV exposure indicates the formation of Au nanoparticles studded into the agarose hydrogel film, i.e., Au@Agr.

Thioglycolic acid (TGA) mediated fragmentation of Au nanoparticles studded into agarose film (TGA-Au@Agr). Au@Agr film was treated with thioglycolic acid (TGA) to investigate any fragmentation of Au NPs studded inside the Agr film. 2% (v/v) of TGA in Milli Q water was prepared, 10 mL of which was poured into a Petri dish containing the Au NPs studded agarose (Au@Agr) film and kept for 1 h at room temperature. The 2%TGA-Au@Agr film was then investigated with UV-visible analysis to ensure any fragmentation of Au NPs by TGA. The UV-visible spectrum was taken in reflectance mode, which is then converted instrumentally to absorbance through the Kubelka–Munk equation. Likewise, cysteine (CS), 2-mercaptoethanol (ME), L-methionine (MET), thiourea (TU), and 1,3-propanedithiol (PDT) were also used to prepare other thiol-fragmented Au@Agr films, and named as 2%CS-Au@Agr, 2%ME-Au@Agr, 2%MET-Au@Agr, 2%TU-Au@Agr, and 2%PDT-Au@Agr, respectively. The complete step-by-step methodical formulation of thiol-Au@Agr film is presented in a pictorial form in Scheme 1. The images of Agr, Au@Agr, and thiol-Au@Agr films prepared in the laboratory are also included in Scheme 1.

Scheme 1 Schematic of the preparation of gold nanoparticles studded into an agarose film (Au@Agr) and the subsequent fragmentation of Au@Agr mediated by a group of thiols. Schematic of each film is also supported by the image of the original films prepared in the laboratory.

Investigation of thiol-Au@Agr film as a catalyst for p-NP reduction

All the thiol-Au@Agr films were investigated for catalytic reduction of p-nitrophenol (p-NP) to p-aminophenol (p-AP) in the presence of sodium borohydride (NaBH₄). For the process, 20 μL of 1 mM p-NP was mixed with 100 mL of 5 mM NaBH₄ solution. The resulting solution showing λ_max at 400 nm due to the formation of p-nitrophenolate ion (p-NP(r)) was then monitored for the complete catalysis study. A small piece of thiol-Au@Agr film (1 × 1 cm, thickness ∼ 0.2 mm) was immersed in 2 mL of p-NP(r) followed by easy stirring, and then the UV-visible spectrum of the p-NP(r) monitoring its 400 nm peak was taken in regular interval of time, viz., 2.5 s, 5 s, 10 s, 15 s up to 60 s.

Characterization

The hydrogel films were characterized by UV-visible spectroscopy (Shimadzu-UV 2600) to study changes in the absorbance pattern of Agr, Au@Agr and thiol-Au@Agr films (each film strip used was sized ∼1 × 1 cm, thickness ∼0.2 mm). The spectra were first obtained in the reflectance mode in the 800–190 nm range, which was then converted into its absorbance value instrumentally through the Kubelka–Munk equation. The surface morphology and fragmentation of Au@Agr were investigated using a scanning electron microscope (SEM) from Carl Zeiss (Sigma VP). The energy dispersive X-ray spectroscopy (EDX) connected to SEM was used to ascertain the elements present in all the thiol-Au@Agr films. For the SEM and EDX analysis, a ∼1 × 1 cm strip was placed on a stub, dried at 50 °C for 1 h and used for imaging. The particle size distribution analyses of freshly prepared Au NPs solution and all thiol-Au solutions were done with a Malvern Zetasizer NanoZS 90.

Results and discussion

Fragmentation of Au@Agr film by selective thiols

An investigation of the proficiency of a group of thiols to mediate a non-conventional fragmentation of Au NPs, which are studded into agarose hydrogel film, is presented here. The peculiar role of agarose in assisting a facile fragmentation of Au NPs and the candidature of such a film in rapid reduction catalysis of p-nitrophenol (p-NP) is also demonstrated. For the study, Au NPs are first studded into the Agr film, as detailed in the experimental section (Scheme 1). Briefly, a 2.5% (w/v) Agr film was treated with 10 mL of 5 mM HAuCl₄ solution, and then Au³⁺ was reduced to Au⁰ by exposing the film to 365 and 254 nm wavelength UV radiation for only 10 min. The completion of the Au³⁺ → Au⁰ reduction process was indicated by ruby red coloration of the Agr film resulting in the formation of an Au NPs studded Agr film (Au@Agr). The formation of the Au@Agr film was experimentally proved by UV-visible spectrophotometry analysis. In the UV-vis spectrum of Au@Agr film, a sharp peak at a wavelength 540 nm is distinguishably observed suggesting the successful formation of Au NPs of size 40–60 nm with UV exposure of Au³⁺ loaded Agr (Fig. 1). A distinct variance in the spectral patterns of the Agr and Au@Agr films was also observed through UV-visible analysis. Furthermore, the actual loading of Au³⁺ into the Agr film was checked and found to be 45% (Fig. S1, ESI†). An excess loading of Au³⁺ above 45% resulted in the formation of aggregated Au NPs of sized >60 nm when subjected to UV treatment. Therefore, this loading capacity was taken as the standard for the complete study.

Fig. 1 The UV-visible spectrum of agarose (Agr) and Au@agarose (Au@Agr) hydrogel film.

After ascertaining the successful formation of Au NPs studded into the Agr film (Au@Agr) through UV-visible analysis, the feasibility of interaction of thiols towards such Agr supported Au NPs was studied. A group of thiols selected for the study includes thioglycolic acid (TGA), cysteine (CS), 2-mercaptoethanol (ME), L-methionine (MET), thiourea (TU), and 1,3-propanedithiol (PDT). Each thiol was allowed to interact with Au@Agr film for 1 h at room temperature after which the film was analysed through UV-visible spectroscopy to check any perceivable changes in the spectral pattern of Au@Agr. Interestingly, it is observed that after interaction with thiol, the Au@Agr film showed a blue shift in the SPR peak of Au NPs observed earlier in its UV-visible spectrum. However, such kind of blue shift was found only in case of Au@Agr treated with TGA, CS, ME and MET. In TGA-Au@Agr, the blue shift observed in the SPR peak of Au NPs was 7 nm from 540 nm → 533 nm. In CS-Au@Agr, this shift was from 545 nm → 539 nm, a 6 nm shift. Moreover, in ME–Au@Agr and MET–Au@Agr, an 11 nm blue shift in both was observed showing 540 nm → 529 nm & 545 nm → 534 nm shifts, respectively. The PDT and TU treated Au@Agr, i.e., PDT-Au@Agr and TU-Au@Agr, respectively, failed to show any such blue shift in the SPR peak of Au NPs. Instead, a 14 nm red shift (534 nm → 548 nm) was observed in PDT-Au@Agr, and in TU-Au@Agr, no shift was evident. This peculiar behaviour of different thiols towards Au@Agr and also the selectivity of thiols to show blue or red shifts of the SPR peak of Au NPs studded into Agr (Au@Agr) detected by a UV-visible spectrophotometer is shown in Fig. 2.

Fig. 2 The UV-visible spectra showing the blue/red shift in SPR peak of Au NPs studded into agarose film (Au@Agr). (A) 2%TGA-Au@Agr, (B) 2%CS-Au@Agr, (C) 2%ME-Au@Agr, (D) 2%TU-Au@Agr, (E) 2%PDT-Au@Agr, and (F) 2%MET-Au@Agr.

It is well known that a blue/red shift in the SPR peak of a metal nanoparticle is an optical manifestation of a significant change in its particle size.^24,25 In order to further examine this change in the particle size of Au@Agr on binding with thiols, a systematic scanning electron microscopy (SEM) analysis is, done before and after treating Au@Agr with thiols. As illustrated in the SEM images (Fig. 3), the change in particle size of Au@Agr (50–60 nm) on treatment with thiols is vividly perceptible in accordance with the blue shifts of the SPR peak in the UV-visible spectrum of Au@Agr. In the SEM images of TGA-Au@Agr and CS-Au@Agr, a clear dense distribution of Au NPs having a particle size ≤30 nm is observed, contrary to the sparsely distributed Au NPs of size ≤30 nm in ME-Au@Agr as well as in MET-Au@Agr.

Fig. 3 Scanning electron microscopy (SEM) images of Au NPs studded into the agarose film (Au@Agr), 2%TGA-Au@Agr, 2%CS-Au@Agr, 2%ME-Au@Agr and 2%MET-Au@Agr showing fragmentation of Au NPs particle size after thiol treatment.

It would be informative to have a clear picture of Au NPs size distribution in all the films simply by assessing their SEM images. As shown in Fig. 4, in Au@Agr, a very broad particle size distribution with Au NPs ranging from 30 to 60 nm was obtained. In TGA-Au@Agr, the observation of very small and densely distributed fragmented Au NPs depicts the highest density of <10 nm Au NPs with fewer numbers of <20 nm AuNPs, and in CS-Au@Agr, the Au NPs density is highest in the particle size range of 11–20 nm with comparatively lesser density in the particle size range of 1–10 nm. On the other hand, the sparse distribution of Au NPs in ME-Au@Agr shows particles in 11–20 nm size range with very few in 21–30 nm size range. The Au NPs in this similar size range is also determined in MET-Au@Agr, but the number of particles is further less in support of their sparser distribution shown in the SEM image. For PDT-Au@Agr and TU-Au@Agr, the other thiols of the group, no sign of fragmentation is evident in their SEM images (Fig. S1, ESI†). In fact, bigger sized Au NPs (60–70 nm) were seen in PDT-Au@Agr, supplementing the 14 nm red shift in SPR peak of Au NPs observed in its UV-visible spectrum. Moreover, a distribution of 50–60 nm sized Au NPs in the SEM image of TU-Au@Agr is observed confirming no shifting of the SPR peak of Au@Agr upon binding with TU, which was detected in their UV-visible spectra.

Fig. 4 The particle size distribution plot assessed from SEM images (Fig. 3) of Au@Agr, 2%TGA-Au@Agr, 2%CS-Au@Agr, 2%ME-Au@Agr and 2%MET-Au@Agr.

The peculiar and intriguing behaviour of thiols to successfully fragment supported Au NPs (Au@Agr) can be credited to electron ejection on Au NPs by chemisorbed thiols.^15,39 A shift in the SPR band wavelength caused by an electron donor or acceptor in close proximity can be comprehended due to alterations in the density of free electrons available on the nanoparticle. These alterations in turn cause difference in SPR energy making it spectroscopically perceptible. Thiol being a strong nucleophile can easily donate electron density to Au NPs, which accumulates at their surface.^15,40,41 The stabilisation of a nanoparticle strongly depends on the surface charge density which upon reaching a critical value causes excess surface tension on the nanoparticle.^14,15,42 When the surface charge density exceeds this critical value, increased disturbance leads to fragmentation of the nanoparticle as witnessed in TGA-Au@Agr, CS-Au@Agr, ME-Au@Agr and MET-Au@Agr. A lesser density of Au NPs in ME-Au@Agr and MET-Au@Agr can be attributed to the presence of terminal –OH groups in ME and –S–CH₃ groups in MET. It is reported that presence of –SH as well as –COOH groups together is the crucial factor for effective fragmentation of Au NPs, where –SH group plays the role of an electron donor and –COOH group aids in hydrogen bonding between adsorbed thiols and free thiols contributing to local concentration around the Au NPs.¹⁵ Due to the absence of one favouring factor among these two in ME and MET, a lower degree of fragmentation processes results in reduced density of fragmented Au@Agr. The contradictory behaviour of no fragmentation observed in TU-Au@Agr and PDT-Au@Agr can also be accounted for by the lack of suitable functional groups. TU does not have a terminal S-atom making donation of free electrons to Au NPs difficult, and also the presence of the NH₂ group at both ends makes interaction further difficult.¹⁵ Thus, TU-Au@Agr does not show any shift of the Au@Agr SPR peak. PDT-Au@Agr, on the other hand, shows a 14 nm red shift of SPR peak of Au@Agr which might be due to lowering of electron density on Au NPs by PDT resulting in aggregation and a red shift.⁴⁰ Therefore, the experimental analyses by UV-visible spectroscopy and SEM imaging provide significant legitimate observations confirming the fact that thiols (TGA, CS, ME and MET) are capable of playing the chemistry with supported Au NPs (Au@Agr) in a non-conventional way, resulting in successful fragmentation of these supported Au NPs.

The role of agarose (Agr) as a support to Au NPs, and a favourable condition for fragmentation as well was inquired about. The reduction of Au³⁺ → Au⁰ by UV exposure in only 10 min was made possible by Agr without adding any reducing agent. However, a UV exposure to a solution of Au³⁺ ions could not reduce it to Au⁰ even after 5 h. This clearly indicates the role of Agr as a stimulating condition for obtaining Au NPs simply by quick UV exposure. In addition, to check whether the fragmentation feasibility of thiols works for Au NPs in a solution also (i.e., when Au NPs are not supported by Agr), another investigation based on dynamic light scattering (DLS) was done. Herein, the fragmentation of freshly prepared Au NPs in solution by the group of thiols (TGA, CS, ME, MET) was investigated. For the analysis, a 0.1% solution of each thiol was prepared, and then mixed with freshly prepared Au NPs (2 mL of 0.1% thiol solution + 40 μL of Au NPs solution) followed by DLS investigation for all the samples (Fig. S3, ESI†). Excitingly, the thiol solutions could fragment the 60 nm sized Au NPs up to a maximum of 35 nm sized particles only, which was observed in case of CS-Au. For ME-Au and MET-Au, the Au NPs sizes observed were 38 nm and 40 nm, with TGA-Au showing minimum fragmentation reporting 57 nm sized Au NPs. Clearly, the fragmentation by thiols is less feasible if the Au NPs are present in the solution as the UV-visible and SEM studies (discussed before) clearly show that fragmentation of Au NPs are efficient when Agr acts as a solid support. This is probably because Agr restricts the movement of thiol molecules as well as Au NPs empowering an easy and effective electron density transfer leading to their successful fragmentation into much smaller sizes.

Synergistic role of thiols and Au@Agr for catalytic reduction of p-nitrophenol

Understanding of non-conventional chemistry occurring between the thiols and Au@Agr led to another exploration of the viability of the catalytic property of Au NPs. Even after fragmentation of Au NPs, the residence of excess electron density on their surface is quite logical. Therefore, utilization of this electron density for effective and rapid reduction of one of the most refractory pollutant, p-nitrophenol (p-NP), to p-aminophenol (p-AP) was the next goal in this study.

An aqueous solution of p-NP has a characteristic maximum absorption peak appearing sharply at 320 nm in its UV-visible spectrum. Upon addition of freshly prepared NaBH₄ solution in large excess, the 320 nm peak red shifts to 400 nm due to the formation of p-nitrophenolate ion in the alkaline surrounding triggered by the addition of NaBH₄. When unattended by a catalyst, this 400 nm peak of p-nitrophenolate ion stays stable with no sign of thermodynamically favoured reduction to p-AP (the reported standard reduction potential of p-nitrophenol/p-aminophenol is −0.76 V and that of H₃BO₃/BH₄⁻ is −1.33 V). As mentioned in various reports,^25–29 this p-nitrophenolate ion peak does not alter even after several days, indicating that the conversion of p-nitrophenolte to p-AP is a very sluggish process.

With an intent to catalyse this reduction reaction of p-NP, a small strip of 2%TGA-Au@Agr film (size ∼1 × 1 cm, thickness ∼ 0.2 mm) was inserted into a 2 mL solution of p-NP(r) (p-NP(r) = a stock solution prepared by mixing freshly prepared 20 μL of 1 mM p-NP and 100 mL of 5 mM NaBH₄ solutions showing λ_max = 400 nm). Amazingly, an instant decolorization of the p-NP(r) solution within a few seconds of time was noted as the first observation. Therefore, to observe precisely, another fresh strip of 2%TGA-Au@Agr film was inserted again, and the noted change in the colour of p-NP(r) solution was then examined every 5 s by UV-visible spectroscopy (Fig. 5). It is very exciting to observe that in the presence of 2%TGA-Au@Agr, the sluggish peak at 400 nm corresponding to p-nitrophenolate ion showed a complete decrease in only 20 s of time with an emerging peak appearing around 300 nm substantiating the formation of p-AP. Moreover, since the reaction is monitored every 5 s, the presence of any Au NPs peak would have been seen in the UV-visible spectrum of p-NP(r), if any leaching of Au NPs from the film to the p-NP(r) solution would have occurred, which validates that the catalytic reduction reaction is progressing only on the surface of the 2%TGA-Au@Agr film. The same catalytic reduction analysis of p-NP(r) solution was examined again using other films, viz., 2%CS-Au@Agr, 2%ME-Au@Agr, and 2%MET-Au@Agr (2%TU-Au@Agr and 2%PDT-Au@Agr were discarded as they do not show any fragmentation of Au@Agr).

Fig. 5 The UV-visible absorption spectral analysis showing catalysis of the reduction reaction of p-nitrophenol (p-NP) to p-aminophenol (p-AP) using 2%TGA-Au@Agr, 2%CS-Au@Agr, 2%ME-Au@Agr and 2%MET-Au@Agr as catalysts. p-NP(r) is p-nitrophenol ions produced by adding freshly prepared excess solution of NaBH₄ to p-NP solution.

An instant catalysis for complete reduction of p-NP(r) to p-AP was observed in the presence of 2%CS-Au@Agr in only 30 s of time, as shown in Fig. 5, featuring degradation of 400 nm peak of p-NP(r) accompanied by occurrence of a new 300 nm peak for p-AP. Contradictorily, even after the presence of fragmented Au@Agr, 2%ME-Au@Agr and 2%MET-Au@Agr were found to be incapable of showing effective reduction reaction catalysis for p-NP(r) to p-AP even after 10 min (Fig. 5). This discrepancy was expected because of the presence of a much lower number of Au NPs in 2%ME-Au@Agr as well as in 2%MET-Au@Agr, as is clearly evident from their SEM images shown in Fig. 3.

The reduction reaction of p-NP to p-AP is considered to be a pseudo-first order reaction owing to presence of NaBH₄ in large excess.^26–37 As shown in Fig. 6(A), a graph of ln(C_t/C₀) is plotted against time to study the reaction when catalysed by all the different catalytic films, viz., 2%TGA-Au@Agr and 2%CS-Au@Agr. Herein, C_t denotes the concentration of p-NP(r) at time ‘t’ seconds in the presence of the catalytic film, and C₀ is the concentration of p-NP(r) at time ‘zero’ seconds. The value of each concentration was calculated from the observed values of absorbance at λ_max = 400 nm after every 5 s during p-NP(r) catalytic cycling. Upon plotting ln(C_t/C₀) values vs. time, a linear relationship is observed for both the p-NP catalytic reactions catalysed by 2%TGA-Au@Agr and 2%CS-Au@Agr. The rate constant obtained from linearly fitted curve reports a value of 1.6 × 10⁻¹ s⁻¹ and 1.1 × 10⁻¹ s⁻¹ for 2%TGA-Au@Agr and 2%CS-Au@Agr catalysed p-NP reduction reactions, respectively. To the best of our knowledge, this rate constant value of the order of 10⁻¹ s⁻¹ is one of the fastest rate constant shown by a solid supported catalyst for the p-NP reduction reaction to date. For a comparative study, the p-NP reduction catalytic reaction was also studied using Agr and Au@Agr film as catalysts. As shown in Fig. S4 (ESI†), the catalytic behaviour of Agr and Au@Agr is insignificant with stagnant rate constant values of 0.01 × 10⁻¹ s⁻¹ and 0.05 × 10⁻¹ s⁻¹ when compared to 2%TGA-Au@Agr and 2%CS-Au@Agr.

Fig. 6 A comparative plot of ln(C_t/C₀) vs. time showing a pseudo-first order rate of reduction reaction of p-NP to p-AP catalysed by (A) 2%TGA-Au@Agr and 2%CS-Au@Agr films and (B) 4%TGA-Au@Agr and 4%CS-Au@Agr films. All graphs display linearly fitted values.

With an idea to check whether the concentration of thiol while preparing the films has anything to do with the catalysis rate, next catalytic films using a higher concentration of thiol were prepared (which is 4%, earlier it was 2%). From Fig. 6(B), it is quite clear that 4%TGA-Au@Agr and 4%CS-Au@Agr films show a relatively faster catalysis of p-NP reduction reaction with respective rate constant values of 6.4 × 10⁻¹ s⁻¹ (an approximately four times higher value) and 2.2 × 10⁻¹ s⁻¹ (an approximately two times higher value). The complete UV-visible analysis of p-NP reduction by 4%TGA-Au@Agr and 4%CS-Au@Agr is shown in Fig. S5, ESI.† Thus, an increase in thiol concentration does show a faster catalytic rate for the p-NP reduction reaction. A comparison table of different rate constant values obtained for the p-NP reduction to p-AP when catalysed by different films prepared is shown in Table 1. Another table showing the comparison between the rate constant values of reduction reaction of p-NP to p-AP when catalysed by different materials reported in the literature and our proposed film is also included in Table 2. Followed by this, a reusability test on 2%TGA-Au@Agr, 4%TGA-Au@Agr, 2%CS-Au@Agr, and 4%CS-Au@Agr was also conducted (Fig. S6, ESI†). For the study, a fresh strip of thiol-Au@Agr film (1 × 1 cm, thickness ∼ 0.2 mm) was taken and the 1^st catalytic cycle of the reduction reaction of p-NP was carried out. After the completion of 1^st catalytic cycle, the film strip was recovered from the solution, washed several times with Milli Q water, dried on a filter paper and then used again for the 2^nd catalytic cycle. It was observed that the activity of 2%TGA-Au@Agr and 2%CS-Au@Agr decreases after its first use, but 4%TGA-Au@Agr was equally effective as a catalyst in its second use; whereas, 4%CS-Au@Agr was found to show relatively lower activity in its second use. A SEM image (Fig. S7, ESI†) of the used catalytic film was taken, which shows that there are assemblies of aggregated Au NPs in the used film, which was otherwise absent in the fresh thiol-Au@Agr film. The aggregation of Au NPs in the film after its first catalytic cycle could be due to the presence of excess NaBH₄ in the p-NP(r) catalytic reaction solution.⁴³ This aggregation of Au NPs probably caused a decrease in the catalytic activity of the film after the 1^st use.

Table 1 Comparison of rate constant values of p-NP reduction reaction catalysed by all the different films as catalysts

Film	Thiol	% of thiol	Rate constant (k)
Agr	None	0	0.01 × 10^−1 s^−1
Au@Agr	None	0	0.05 × 10^−1 s^−1
2%TGA-Au@Agr	Thioglycolic acid	2	1.6 × 10^−1 s^−1
4%TGA-Au@Agr	Thioglycolic acid	4	6.4 × 10^−1 s^−1
2%CS-Au@Agr	L-Cysteine	2	1.1 × 10^−1 s^−1
4%CS-Au@Agr	L-Cysteine	4	2.2 × 10^−1 s^−1
2%ME-Au@Agr	2-Mercaptoethanol	2	3.3 × 10^−4 s^−1
2%MET-Au@Agr	L-Methionine	2	1.6 × 10^−5 s^−1

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Table 2 The comparison table between the rate constant values of reduction reaction of p-NP to p-AP when catalysed by different materials reported in the literature and our proposed film (4%TGA-Au@Agr and 4%CS-Au@Agr)

S. No.	System	Rate constant (k)	Time taken	Reference
1	4%TGA-Au@Agr film (in solid phase)	6.4 × 10^−1 s^−1	5 s	Our proposed material
2	4%CS-Au@Agr film (in solid phase)	2.2 × 10^−1 s^−1	15 s	Our proposed material
3	Platinum nanoparticles stabilised by guar gum (in solution)	8.3 × 10^−3 s^−1	420 s	S. Pandey and S. B. Mishra, Carbohydrate Polymers, 2014, 113, 525–531
4	Au(0)@TpPa-1 (in solution)	5.35 × 10^−3 s^−1	240 s	P. Pachfule, S. Kandambeth, D. Díaz Díaz and R. Banerjee Chem. Commun., 2014, 50, 3169–3172
5	Ellipsoid-like MCM-41 supported Ag nanocomposite (in solution)	4.3 × 10^−3 s^−1	180 s	N. Hao, L. Li and F. Tang, J. Mater. Chem. A, 2014, 2, 11565–11568
6	SiO2HP/AuNP composite particles (in solution)	11.3 × 10^−3 s^−1	500 s	H. Gu, J. Wang, Y. Ji, Z. Wang, W. Chenab and G. Xue, J. Mater. Chem. A, 2013, 1, 12471–12477
7	Fe3O4@TiO2/Au@Pd@TiO2(in solution)	1.06 min^−1	4 min	W. Hu, B. Liu, Q. Wang, Y. Liu, Y. Liu, P. Jing, S. Yu, L. Liua and J. Zhang, Chem. Commun., 2013, 49, 7596–7598
8	Au1-1 h (in solution)	6.84 × 10^−4 s^−1	3000 s	S. Fazzini, D. Nanni, B. Ballarin, M. C. Cassani, M. Giorgetti, C. Maccato, A. Trapananti, G. Aquilanti and S. I. Ahmed, J. Phys. Chem. C, 2012, 116, 25434–25443
9	Ag(0.7)–SBA-15 composites (in solution)	9.0 × 10^−3 s^−1	360 s	J. Han, P. Fang, W. Jiang, L. Li and R. Guo, Langmuir, 2012, 28, 4768–4775
10	Au NPs prepared from 0.5 mL Breynia rhamnoides (in solution)	4.65 × 10^−3 s^−1	600 s	A. Gangula, R. Podila, Ramakrishna M, L. Karanam, C. Janardhana, and A. M. Rao, Langmuir, 2011, 27, 15268–15274
11	AuNPs/SNTs nanocomposite (in solution)	10.64 × 10^−3 s^−1	280 s	Z. Zhang, C. Shao, P. Zou, P. Zhang, M. Zhang, J. Mu, Z. Guo, X. Li, C. Wanga and Y. Liua, Chem. Commun., 2011, 47, 3906–3908
12	α-Cyclodextrin capped Au nanoparticles (in solution)	4.65 × 10^−3 s^−1	600 s	T. Huang, F. Meng, and L. Qi, J. Phys, Chem, C, 2009, 113, 13636–13642
13	AuNPs–PPl dendrimer (in solution)	13.2 × 10^−3 s^−1	200 s	K. Hayakawa, T. Yoshimura, and K. Esumi, Langmuir, 2003 19, 5517–5521

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An explicit insight into the mechanistic behaviour for the catalysis reaction by the films was done (Scheme 2). The reduction catalysis starts only after adsorption of p-NP and NaBH₄ on the surface of Au NPs. In the present case, each component Agr, Au@Agr and TGA-Au@Agr or CS-Au@Agr plays a specific and favouring role in aiding a faster catalytic reduction of p-NP. Agr: (i) the process of adsorption of reactants made using an Agr hydrogel film as a solid support (ii) once adsorbed, the solid support also limits the movement of the adsorbed reactants making the electron transfer process swift. Au@Agr: (i) it provides a perfect and aligned surface with extra electrons for the catalysis reaction to play on with a swift relay of electrons from BH₄⁻ to p-NP; (ii) supported Au NPs also dilute the chance for aggregation, which is well known to be harmful for a catalysis reaction. TGA-Au@Agr or CS-Au@Agr: (i) first and foremost, TGA and CS prosecute the fragmentation of Au NPs delivering much smaller nanoparticles on a solid support for catalysis; (ii) second, the presence of TGA or CS stimulates adsorption of p-NP much easily due to the probability of forming hydrogen bonding between them; (iii) third, such interactions in conjunction with the agarose hydrogel nature make maximum and quick adsorption of p-NP for effective and swift catalysis. Hence, all these insights give a clear picture of the synergistic roles of Agr, Au@Agr and TGA/CS for such an efficient catalytic activity towards p-NP to p-AP reduction reaction in just ∼20 to 30 seconds.

Scheme 2 Schematic of the fragmentation of Au@Agr by thiols (TGA), and the synergistic role of TGA and Au@Agr to catalyse the reduction reaction of p-nitrophenol (p-NP).

Conclusion

This study presents a physical experimentation and demonstration of the capability of a group of thiols to fragment an Au@Agarose film in the absence of any auxiliary applied energy, an unexplored non-conventional chemistry between thiols and supported Au NPs. A group of thiols (TGA, CS, ME and MET) were found to be very proficient in fragmenting 50–60 nm sized Au@Agr into well dispersed ≤30 nm sized nanoparticles. The fragmentation process can be easily explained by transfer of electron density from thiols to Au@Agr causing an excess of charge on Au NPs, thereby undergoing fragmentation to relieve the surface tension. The TGA-Au@Agr and CS-Au@Agr films studded with such fragmented Au NPs have proven to be an efficient catalytic material for rapid reduction of p-NP to p-AP in just ∼20 to 30 s. The rate constants for 2%TGA-Au@Agr and 2%CS-Au@Agr catalysed p-NP reduction are found to be 1.6 × 10⁻¹ s⁻¹ and 1.1 × 10⁻¹ s⁻¹, respectively. A faster catalytic rate for the reduction reaction is evident for increasing the concentration of TGA as well as CS with two reuses of the same film strip. Such a fast catalysis by TGA-Au@Agr and CS-Au@Agr can be accounted for from a synergistic role of Agr, Au@Agr and TGA or CS complementing each others role and contributing towards an efficient and rapid catalytic behaviour. On all these notes, it can concluded that this study opens up new insights about the chemistry of thiols towards supported Au NPs as well as the effectiveness of such a system to serve as an amazing catalyst for industrial noxious and lethargic pollutants.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

The authors would like to thank the Science and Engineering Research Board (SERB), New Delhi, for project grant SB/S1/PC-69/2012 and the BRNS, Mumbai, Grant No. 34/14/20/2014-BRNS. N. G. wants to thank CSIR, New Delhi, for fellowship.

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Footnotes

† Electronic supplementary information (ESI) available: The UV-visible study to determine loading of Au³⁺ into Agr film. The SEM images of PDT-Au@Agr and TU-Au@Agr. DLS analysis of Au NPs solution, TGA-Au, CS-Au, ME-Au, MET-Au in solution. The comparative UV-visible spectra and plot of ln(C_t/C₀) vs. time showing p-NP reduction by Agr, Au@Agr, TGA-Au@Agr, CS-Au@Agr as catalyst. The UV-visible spectra of p-NP reduction reaction catalysed by 4%TGA-Au@Agr and 4%CS-Au@Agr. The UV-visible spectra for reusability study. The SEM image of used 2%TGA-Au@Agr film. The molecular structure of TGA, CS, ME, MET, TU and PDT. See DOI: 10.1039/c5ra19567e

‡ The manuscript is written through contributions of all authors.

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