Aminothiols Sensing Based on Fluorosurfactant-Mediated Triangular

Jian Zhang , Jianping Wang , Liang Yang , Bianhua Liu , Guijian Guan , Changlong Jiang , Zhongping Zhang. Chem. Commun. 2014 50 (100), 15870-15873 ...
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Aminothiols Sensing Based on Fluorosurfactant-Mediated Triangular Gold Nanoparticle-Catalyzed Luminol Chemiluminescence Qianqian Li,† Fang Liu,† Chao Lu,*,† and Jin-Ming Lin‡ † ‡

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Department of Chemistry, Tsinghua University, Beijing 100084, China

bS Supporting Information ABSTRACT: A novel method was used to synthesize triangular gold nanoparticles (AuNPs) (i.e., trisodium citrate reduction of HAuCl4 in the presence of nonionic fluorosurfactant). The asprepared triangular AuNPs owned higher surface-to-volume ratio and more active surface sites compared to spherical AuNPs, which facilitated the active oxygen intermediates generation and electron-transfer processes on the surface of triangular AuNPs. Therefore, it was first found that triangular AuNPs displayed greater catalytic activity (ca. 125-fold) toward luminol chemiluminescence (CL) than spherical AuNPs. More interestingly, ultratrace aminothiols (ca. 0.1 nM) can interrupt the formation of the active oxygen intermediates by forming AuS covalent bonds on the surface of triangular AuNPs, resulting in a great decrease in CL intensity, while the other biomolecules including 19 standard amino acids, alcohols, organic acids, and saccharides have no effect on triangular AuNPs-catalyzed luminol CL signals. These significant features of triangular AuNPs-catalyzed luminol CL were the ability to detect aminothiols in the presence of other essential amino acids and biomolecules.

1. INTRODUCTION Fluorosurfactants are a class of surfactants with partially perfluorinated tails which are hydrophobic as well as lipophobic. Recently, they have become a interesting topic due to their higher surface activity and chemically stability than their hydrocarbon analogues in acid or alkaline solutions.13 Nonionic fluorosurfactants (F(CF2CF2)17CH2CH2O(CH2CH2O)015H, Zonyl FSN) are commercially available with a polyoxyethylene chain in their hydrophilic part and a fluorocarbon chain at the hydrophobic part. Zu and co-workers concluded that the adsorption of FSN at hydrophilic electrodes was stronger than that of their hydrocarbon analogues.4,5 Recently, Tang et al. utilized scanning tunneling microscopy (STM) measurement to validate the formation of FSN SAMs on Au surface through physical interaction between its hydroxyl groups at the end of the hydrophilic part.6 Furthermore, anisotropic growth of nanoparticles can also be controlled by a selective adsorption of surfactant molecules on specific crystal planes. Therefore, it can be inferred that an effective liquid/solid interfacial selectively adsorption of FSN on Au surface prevented anisotropic growth, leading to precise morphologies, such as triangle, pentagon, and hexagonal.7 The size/shape preparation of AuNPs in homogeneous and heterogeneous systems is important toward chemistry and physics applications of nanoscale materials. This conclusion stems from the fact that any property within the nanometer regime depends on their size and shape, leading them to exhibit r 2011 American Chemical Society

different properties from the bulk gold.8 The anisotropic growth of AuNPs remains an important and challenging task since gold has a highly symmetric face-centered cubic (fcc) structure and usually tends to afford spherical shape to reduce its surface energy.9 However, metal nanoparticles of various shapes have different catalytic activity for various organic and inorganic reactions.10 Therefore, in order to allow additional shapes other than spheres, the relatively simple surfactants with straight alkyl chain, such as cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS), have generally been used to synthesize and stabilize AuNPs in a micelle or a reversed micelle media, where the surfactants not only impart stability to the clusters but also define the shapes of the nanoparticles due to specific interaction of the surfactants with different crystal faces of the particles during the crystal growth process.1113 In our previous papers,14,15 it was found that FSN-capped spherical AuNPs exhibited excellent stability under high salinity conditions and wide pH ranges and accomplished an extremely selective assay for cysteine and homocysteine based on the aggregation of FSN-capped spherical AuNPs by sensing technology. It is well-known that the dynamic range of the analyteinduced spherical AuNPs aggregation is generally quite narrow Received: January 23, 2011 Revised: May 5, 2011 Published: May 16, 2011 10964

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The Journal of Physical Chemistry C (about 1 order of magnitude) with a detection limit in the higher order (μM).1618 Therefore, it would be necessary to use other detection techniques to make AuNPs suitable for aminothiols sensing with higher sensitivity and wide linear range. The chemiluminescence (CL) technique has been an attractive topic of intensive research because of its absence of unwanted strong background light, low noise signals, better sensitivity, and wide linear dynamic range. It has been reported that spherical AuNPs can enhance luminolH2O2 CL signals, and some compounds containing OH, NH2, and SH groups could inhibit the CL signals from the luminolH2O2spherical AuNPs system, and the detection limits were even as low as 1 nM for these compounds; however, its poor selectivity made it impossible for an accurate analytical measurement.19 Therefore, it will be highly promising to combine the excellent selectivity of the FSN-capped AuNPs for SH groups and high sensitive CL measurement in analytical applications. Unfortunately, FSNcapped AuNPs also showed poor selectivity to SH groups using the luminolH2O2 CL system. In this study, triangular AuNPs with high yield were synthesized by trisodium citrate reduction of HAuCl4 in presence of (0.05%, w/w) FSN ligands. We examined the response of triangular AuNPs for CL intensity of the luminolH2O2 system and compared it with spherical AuNPs synthesized under the same conditions in the absence of FSN. Interestingly, the asprepared triangular AuNPs displayed much stronger catalytic activity in luminol CL than spherical AuNPs. Moreover, the triangular AuNPs-catalyzed luminolH2O2 system owned a high selectivity and sensitivity toward aminothiols. The linear range for aminothiols can reach 3 orders of magnitude, and the limits of detection (S/N = 3) were found to be as low as 0.1 nM. The enhancement mechanism of triangular AuNPs on luminolH2O2 CL reaction and the inhibition mechanism of aminothiols on luminolH2O2triangular AuNPs CL were investigated in detail.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Solutions. Zonyl FSN-100 (F(CF2CF2)17CH2CH2O(CH2CH2O)015H), Brij-35 TritonX-100 and 20 standard amino acids, homocysteine, and glutathione were purchased from Sigma-Aldrich (St. Louis, MO). Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 3 3H2O) and trisodium citrate were purchased from Acros. A 0.01 M stock solution of luminol (3-aminophthalhydrazide) was prepared by dissolving luminol (Acros, Geel, Belgium) in 0.1 M sodium hydroxide solution without purification. The storage of luminol leads to a stabilization of the luminol reactivity, and thus it was used after several weeks. Working solutions of luminol were prepared by diluting the stock solution with deionized water (Milli Q, Millipore, Barnstead, CA). Working solutions of H2O2 were prepared daily from 30% (v/v) H2O2 (Beijing Chemical Reagent Co., Beijing, China). All the reagents were of analytical grade, and all solutions were prepared with deionized water. 2.2. Synthesis of FSN-Capped Spherical AuNPs and Triangular AuNPs. All glassware used for preparation of AuNPs was thoroughly washed with freshly prepared aqua regia (HNO3:HCl = 1:3), rinsed extensively with deionized water, and then dried in an oven at 100 °C for 23 h. The spherical AuNPs with average diameters of 16 nm were prepared following the literature procedure.18 Briefly, a 50 mL solution of 0.02% trisodium citrate

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was brought to a vigorous boil with stirring in a round-bottom flask fitted with a reflux condenser, and then 85 μL of 5% HAuCl4 was added to the stirring and refluxing trisodium citrate solution. The solution was maintained at the boiling point with continuous stirring for 10 min, then the solution was cooled to room temperature with continued stirring, and 0.5 mL of 5% FSN was added (denoted as FSN-capped spherical AuNPs). The suspension was stored at 4 °C until further use. FSN-capped 38 nm spherical AuNPs were prepared by varying the concentration of citrate during the reduction step. Preparation of triangular AuNPs follows the same procedure as 16 nm spherical AuNPs, except that a 50 mL mixture solution containing 0.02% trisodium citrate and 0.05% FSN was brought to a vigorous boiling. We also used 0.05% Triton X-100 or 0.05% Brij-35 to synthesize AuNPs following the same procedure as adopted for triangular AuNPs preparation. Assuming the spherical particles and triangular particles with a density equivalent to that of bulk gold (19.30 g/cm3),18 the concentrations of 16 and 38 nm spherical AuNPs and 32 nm triangular AuNPs were calculated to be ∼1.98, ∼0.15, and ∼1.06 nM. 2.3. Apparatus and Characterization of Triangular AuNPs. UVvis spectra of triangular AuNPs were measured on a USB 4000 miniature fiber-optic spectrometer in absorbance mode with a DH-2000 deuterium and tungsten halogen light source (Ocean Optics, Dunedin, FL). The sizes, shape, and their distribution of spherical AuNPs and triangular AuNPs were confirmed through transmission electron microscope (TEM) measurements using a Hitachi-800 TEM (Tokyo, Japan) and high-resolution transmission electron microscopy (HRTEM) measurements using a JEOL JEM-3010 microscope. The TEM and HRTEM specimens were prepared by depositing an appropriate amount of AuNPs onto the carbon-coated copper grids and microgrid, respectively, and excess solution was wicked away by a filter paper. The grid was subsequently dried in air before measure. X-ray powder diffraction (XRD) patterns of triangular AuNPs (which deposited on a glass substrate) were recorded on a Rigaku (Japan) D/max2500VB2þ/PC X-ray diffractometer equipped with graphite-monochromatized Cu/KR radiation ), using a scanning rate of 10 deg/min in 2θ ranges (λ = 1.540 56 Å from 35° to 90°. The centrifugation of AuNPs was operated on a TGL-16B centrifugal machine (Shanghai Anting Scientific Instrument Factory, Shanghai, China). Atomic force microscopy (AFM) in tapping mode was carried out on a NanoScope IIIa (Digital Instruments Co., Santa Barbara, CA) instrument. Before the AFM measurements, excess FSN was removed from the suspension by centrifugation for 10 min at 13000g (repeated three times). BET specific surface areas and pore volume were determined by nitrogen adsorptiondesorption isotherms at liquid nitrogen temperature with a Thermo Electron Corp. Sorptomatic instrument. The CL detection was conducted on a BPCL luminescence analyzer (Institute of Biophysics, Chinese Academy of Science, Beijing, China). 2.4. Procedures for CL Detection. For the signal comparability, we used the flow system as shown in Figure S1 of the Supporting Information. While investigating the catalytic activity of triangular AuNPs on the luminolH2O2 CL system, the gold colloidal solution was injected into the carrier stream (deionized water) through a 100 μL loop-valve injector, mixed with solution A (deionized water), 2  104 M luminol, and 103 M H2O2 through three-way pieces, maintaining a flow rate of 2.0 mL/min for each flow line. The CL signals were monitored by a photomutiplier tube (PMT) adjacent to the flow CL cell. The signal 10965

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Figure 1. UVvis absorption spectra of 16 nm spherical AuNPs and 32 nm triangular AuNPs. Inset: photograph of spherical AuNPs and triangular AuNPs. Figure 3. HRTEM image of 0.05% FSN synthesized AuNPs. Insets show the interplanar distance of the AuNPs as marked by a white frame shown in the corresponding panel.

Figure 2. TEM images of (a) 16 nm spherical AuNPs, (b) 0.05% FSN synthesized triangular AuNPs, (c) 0.05% Triton X-100 synthesized AuNPs, and (d) 0.05% Brij-35 synthesized AuNPs.

was imported to the computer for data acquisition. When the inhibition effects of aminothiols or other biomolecules on the CL system were investigated, solution A is changed to aminothiols or other biomolecules solution and mixed with gold colloids before reacting with luminol and H2O2 solution. The variation in CL intensity of luminolH2O2triangular AuNPs system resulting from the effects of aminothiols can be described as ΔI = I0  I, where I0 is the CL signal in the absence of aminothiols and I is the CL signal in the presence of aminothiols.

3. RESULTS AND DISCUSSION 3.1. Characterization of Triangular AuNPs. Figure 1 shows the UVvis spectra and photographs of AuNPs obtained in the absence of FSN and presence of 0.05% FSN. The spectrum of AuNPs prepared in the absence of FSN revealed a sharp absorbance peak centered 520 nm with wine-red color, which was characteristic of 16 nm spherical AuNPs through TEM measurement (see Figure 2a). However, when 0.05% FSN was added to a solution of trisodium citrate during the AuNPs preparation by

trisodium citrate reduction of HAuCl4, we obtained a wide surface plasmon resonance (SPR) 35 nm red-shifted absorption peak, and the color of colloidal solution was dark blue. We observed from the TEM image the formation of larger nonspherical AuNPs, such as rodlike and triangular (∼78%), was evident apart from spherical AuNPs. The average diameter calculated from the TEM micrographs was 32.0 ( 5.0 nm (see Figure 2b). However, when 0.05% Triton X-100 or Brij-35 was used instead of 0.05% FSN for AuNPs preparation, the synthesized AuNPs exhibited mostly a spherical shape with an average size of 29 or 15 nm, respectively (see Figure 2c,d). The HRTEM image as shown in Figure 3 was performed to investigate the crystal structure of the as-prepared AuNPs; the lattice fringe of the spherical, rodlike, and triangular was found to be most ∼0.226 nm, which corresponds to the (111) lattice spacing. Also, an XRD pattern of triangular AuNPs deposited on a glass substrate was shown, and it can be seen that the diffraction peak at 38.16° assigned to the (111) lattice plane of fcc Au crystal, showing a superior intensity in the pattern, compared to other diffraction peaks (Figure 4). On the basis of these results, it can be implied that the basal plane of triangular AuNPs was the (111) plane, which was in good agreement with those of HRTEM data. These phenomena can be explained as follows: (1) an increasing interatomic distance or decreasing surface atomic density in the order of (111), (100), and (110) resulted in the decrease in the stability of individual planes of fcc geometry;20 (2) FSN molecules can form SAMs on Au(111) and the SAMs showed excellent chemical stability.6 3.2. Optimization of Triangular AuNPs Synthesis. The concentration of FSN was critical to the morphology of triangular AuNPs. Therefore, the concentrations of FSN as 0.025%, 0.05%, 0.1%, and 0.2% were first studied, when keeping the gold and citrate concentrations fixed. Figure S2 shows TEM images and photographs of triangular AuNPs prepared at various concentrations of FSN. Interestingly, it was observed the average particle size of triangular AuNPs first increases and then decreases with increasing concentrations of FSN. At FSN concentration of 0.05%, bigger triangular AuNPs with a higher proportion of triangular shape were formed; moreover, the as-prepared tri10966

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Figure 4. XRD pattern of triangular AuNPs deposited on a glass substrate. Inset is the table of crystal plane (hkl), the angle of diffraction peak (2θ), lattice spacing (d/nm), and percentage of each peak area (area%).

angular AuNPs made the strongest CL emission of the luminolH2O2 system. The effect of citrate trisodium concentration on the CL signals was also investigated in the range from 0.002% to 0.04%. As shown in Figure S3, when the concentration of citrate trisodium was lower or higher than 0.02%, the CL intensity decreased, and the maximum CL intensity was obtained at 0.02% citrate trisodium. Therefore, 0.02% citrate trisodium concentration was used to synthesize triangular AuNPs. The effect of the boiling time of triangular AuNPs in the course of reaction was also investigated, as shown in Figure S4. The CL intensity increased with the increasing of boiling time, which reaches maximum at 10 min before it started to decrease. This was presumably due to the fact that the proportion of triangular shape decreased with the increase of the boiling time, which can be confirmed through TEM measurement (Figure S5). In conclusion, the optimized conditions for the synthesized of triangular AuNPs were as follows: 0.05% FSN, 0.02% citrate trisodium, and 10 min boiling time. At the optimized reaction conditions, the triangular AuNPs were obtained in highest proportion about 78%, and the yield of the various shaped AuNPs can be seen in Figure S6. 3.3. Enhancement of Triangular AuNPs on Luminol CL. In a flow-injection CL setup, the catalytic effects of the as-prepared triangular AuNPs and FSN-capped 16 and 38 nm spherical AuNPs on the luminolH2O2 CL were investigated. As shown in Figure 5, the as-prepared triangular AuNPs, which was synthesized by the same procedure as 16 nm spherical AuNPs, have the most intense luminol CL signal. However, noncapped and FSN-capped 16 nm spherical AuNPs took no effect on luminol CL; noncapped and FSN-capped 38 nm spherical AuNPs have a slightly enhanced to the luminol CL system, which was almost in conformity with the literature.19 Note that FSN-capped 38 nm spherical AuNPs showed little greater enhancement than noncapped capped 38 nm spherical AuNPs, which is presumably due to the fact that the local microenvironment in micelle media is significantly different from the homogeneous media, resulting in enhancement of CL intensity.21

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Figure 5. CL intensity of luminolH2O2 mixed with different AuNPs. (a) 16 nm noncapped spherical AuNPs; (b) 16 nm FSN-capped spherical AuNPs; (c) 38 nm noncapped spherical AuNPs; (d) 38 nm FSN-capped spherical AuNPs; (e) triangular AuNPs.

Blank experiments with the same concentration with triangular AuNPs synthesis were also carried out including trisodium citrate, HAuCl4, and FSN. The results indicated that no enhancement or quenching effects were observed in the presence of trisodium citrate and FSN; however, 0.25 mM HAuCl4 showed a significant enhancement on the luminol CL. It was reported that the enhancement of HAuCl4 on the CL intensity depends on the forms of gold complexes (AuCl4, Au(OH)Cl3, Au(OH)2 Cl2, Au(OH)3Cl, and Au(OH)4), which was obtained by tuning pH.22,23 In this present system, we investigated the effects of the different forms of gold complexes on the luminol CL. The results showed that Au(OH)2Cl2, Au(OH)3Cl, and Au(OH)4, corresponding to the pH scale ranges from 5.0 to 10.0, did not lead to the enhancement of the luminol CL. Note that the pH of the as-prepared triangular AuNPs was 5.2, and the pH of the CL reaction between triangular AuNPs and luminol was 10.0. Therefore, free HAuCl4 has no effect on the background signal. In conclusion, triangular AuNPs played a predominant role in the CL signal enhancement on the luminol CL. The CL enhancement on the luminolH2O2 system by spherical AuNPs was supposed to originate from the catalysis of AuNPs, which facilitated the radical generation and electrontransfer processes taking place on the surface of the AuNPs.19 It was reported that the catalytic activity of metal nanoparticles depended on their shape, and triangular AuNPs showed a higher catalytic activity because of their high surface-to-volume ratio, electron density, and low activation energy.10,2427 In the present system, we confirmed the CL enhancement mechanism of triangular AuNPs on the luminolH2O2 system by AFM techniques. As shown in Figure 6, the thickness of the triangle AuNPs is 9 ( 3 nm. The surface areas of the 38 nm spherical AuNPs and the as-prepared 32 nm triangular AuNPs were 8.2 and 22.7 m2/g by assuming spherical and triangular AuNPs with a density equivalent to that of bulk gold (19.30 g/cm3).28,29 The BET surface areas (data not shown) are much lower than those estimated from the average particle size due possibly to particle aggregation and inadequate sample.28 Therefore, the high surface areas of the as-prepared triangular AuNPs could result in a significant enhancement on the luminol CL 10967

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Figure 6. (a) AFM micrograph of triangular AuNPs. x, y scale of 500 nm. (b) Section analysis of the height and length profile of the triangle AuNPs.

Figure 7. UVvis absorption spectra of triangular AuNPs and the spectral changes of triangular AuNPs in responding to (a) added 10 μM cysteine, and then added 50 μM Pb2þ, and (b) added 10 μM histidine, and then added 50 μM Pb2þ.

intensity compared to spherical AuNPs (Figure 5). This similar conclusion was reported by Li and co-workers, where they reported the catalytic efficiency of the irregular AuNPs on luminol CL to be 100-fold greater than that of spherical AuNPs.30 3.4. Mechanism Discussion. The usual formation mechanism of triangular AuNPs can be formulated as follows: the trivalent gold from HAuCl4 was reduced to Au(I) by the oxidation of trisodium citrate to dicarboxy acetone; Au(I) formed a multimolecular complex with dicarboxy acetone; the complex disproportionated to form gold atoms, and then these atoms adsorbed Au(I) to form large aggregates; further disproportionation leaded to the formation of the larger aggregation of gold atoms.3133 However, as a template, FSN molecules inhibited this association by the formation of SAMs on Au(111) plane through physical interaction between its hydroxyl group at the end of the hydrophilic part and Au substrate.6,34 Therefore, the effective liquid/solid interfacial selectively adsorption of FSN on Au surface prevented anisotropic growth, leading to the formation of the precise morphologies, such as triangle, pentagon, and hexagonal. When the growth rate of the (111) or (100) plane may be inhibited due to the preferential adsorption of FSN, triangular AuNPs were produced.35 In the spherical AuNPs-catalyzed luminol CL reaction, more oxygen-related radicals (OH•, O2•, and other radical derivatives) were generated due to H2O2 absorbed on the surface of AuNPs via the reversible generation of OO bond.36,37 Organic compounds containing OH, NH2, or SH groups can interact readily with AuNPs, thus resulting in a decrease in CL intensity of the luminolH2O2AuNPs system.19 However, we previously reported that FSN ligands allowed AuS interaction

while prohibiting the binding with other functional groups on the surface of AuNPs.14,15 In our case, aminothiols can bind tightly at the active site of the as-prepared triangular AuNPs, leading to oxygen-related radicals cannot easily be generated from H2O2 on the surface of triangular AuNPs. As a result, aminothiols can greatly inhibit triangular AuNPs-catalyzed luminol CL signals (Figure S7). In order to further verify the above speculation, we investigated the UVvis absorption spectra changes of the as-prepared triangular AuNPs in the presence of 10 μM cysteine or histidine. The results showed that no spectral change occurred when cysteine or histidine was alone added into triangular AuNPs solution. However, when 50 μM Pb2þ was added into the cysteine-triangular AuNPs solution, the UVvis absorption spectra changed as shown in Figure 7a; while 50 μM Pb2þ was added into the histidine-triangular AuNPs solution, there was no spectral change (Figure 7b). We reasoned that SH groups can easily get attached to the surface of triangular AuNPs through AuS bonds; the carboxyl groups of aminothiols were exposed on the outer surface of triangular AuNPs. Therefore, the aggregation of triangular AuNPs through electrostatic interaction between carboxyl groups and Pb2þ may have occurred.38 3.5. Aminothiol Sensing. Recently, spherical AuNPs have been found to catalyze the different CL systems, such as the luminolH2O2 system, bis(2,4,6-trichlorophenyl) oxalateH2O2 system, and KIO4NaOHNa2CO3 system.19,3941 The results demonstrated that spherical AuNPs-catalyzed CL had great potential in analytical application. However, the real application of spherical AuNPs-catalyzed CL was limited because of its poor selectivity. Therefore, in order to improve the selectivity of AuNPs, 10968

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During the preparation of triangular AuNPs, FSN ligands acted as a stabilizing and templating agent. On the basis of the data of XRD and HRTEM, we confirmed that the basal plane of triangular AuNPs was (111) plane, and FSN molecules formed SAMs on the as-prepared triangular AuNPs surface. Moreover, the as-prepared triangular AuNPs was found to significantly enhance the CL intensity for the luminolH2O2 system compared to spherical AuNPs synthesized in the absence of FSN. Finally, aminothiols can significantly inhibit the triangular AuNPs-catalyzed luminol CL reaction. Therefore, the triangular AuNPs-catalyzed luminol CL system exhibited its perspective in analytical applications for aminothiols sensing.

’ ASSOCIATED CONTENT Figure 8. CL inhibition of the luminolH2O2triangular AuNPs system by adding different concentration of glutathione. Inset: calibration curve for standard glutathione, ΔI = I0  I, shows the effect of glutathione on the CL intensity of the system, where I0 stands for the signal in the absence of glutathione and I stands for the signal in the presence of glutathione.

the modification of AuNPs’ properties was of greater interest. In this study, we have tried to use FSN-capped spherical AuNPs as a specific probe for SH groups but was unsuccessful as every amino acid can inhibit the CL signal of the luminolH2O2FSN-capped spherical AuNPs system. However, we used the as-prepared triangular AuNPs as a simple and effective amplified catalytic luminol CL label to study the inhibition effects of amino acids and other biomolecules, as we expected it to be useful in bioanalytical applications. We investigated the CL changes of the triangular AuNPs-catalyzed luminol reaction upon the addition of a variety of biomolecules including three aminothiols (i.e., cysteine, homocysteine, and glutathione), 19 standard amino acids, alcohols, organic acids, and saccharides. Figure S8 shows that, upon the addition of 1 μM cysteine, homocysteine, or glutathione, the CL intensity greatly decreased; however, no CL change occurred in the presence of the mixed amino acids, alcohols, organic acids, and saccharides. The specific responses of the as-prepared triangular AuNPs toward aminothiols suggested that FSN ligands allowed thiolgold interaction while prohibiting the binding of other functional groups on the surface of triangular AuNPs.14,15 The analytical performances of the inhibition effects of cysteine, homocysteine, and glutathione on the proposed triangular AuNPs-catalyzed luminol CL were explored by a flow injection procedure. The linear range and detection limits for cysteine, homocysteine, and glutathione are presented in Table S1. The linear ranges for these three compounds reached almost 3 orders of magnitude, and the limits of detection were found to be as low as 0.1 nM, which is about 23 orders of magnitude lower than the usually reported methods, such as electrochemical,42 fluorometric,43 and spectrophotometric.15 The CL inhibition of the triangular AuNPs-catalyzed luminol reaction by adding different concentration of aminothiols has been shown in Figure 8 (glutathione) and Figure S9 (cysteine and homocysteine). The results demonstrated that the triangular AuNPscatalyzed luminol reaction can be developed for aminothiols assay with high selectivity and sensitivity.

4. CONCLUSIONS In conclusion, this paper described a one-step synthesis of triangular AuNPs by citrate reduction of HAuCl4 in FSN solution.

bS

Supporting Information. Schematic diagram of the flow injection CL detection system; TEM images and photographs of triangular AuNPs prepared at different concentration of FSN; effect of citrate trisodium concentrations and boiling time on the CL signals; the yield of different shaped AuNPs; schematic illustration of the proposed CL system; the CL inhibition of different compounds; and the analytical performance of the proposed CL system. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: þ86 10 64411832; e-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 20975010 and 21077008). ’ REFERENCES (1) Macakova, L.; Nordstierna, L.; Karlsson, G.; Blomberg, E.; Furo, I. Langmuir 2007, 23, 771–775. (2) Tan, B.; Lehmler, H.-J.; Vyas, S. M.; Knutson, B. L.; Rankin, S. E. Chem. Mater. 2005, 17, 916–925. (3) Rojas, O. J.; Macakova, L.; Blomberg, E.; Emmer, A.; Claesson, P. M. Langmuir 2002, 18, 8085–8095. (4) Li, F.; Zu, Y. B. Anal. Chem. 2004, 76, 1768–1772. (5) Chen, Z. F.; Zheng, H. Z.; Lu, C.; Zu, Y. B. Langmuir 2007, 23, 10816–10822. (6) Tang, Y. A.; Yan, J. W.; Zhou, X. S.; Fu, Y. C.; Mao, B. W. Langmuir 2008, 24, 13245–13249. (7) Bakshi, M. S.; Sachar, S.; Kaur, G.; Bhandari, P.; Kaur, G.; Biesinger, M. C.; Possmayer, F.; Petersen, N. O. Cryst. Growth Des. 2008, 8, 1713–1719. (8) Jana, N. R.; Gearheart, L.; Murphy, C. J. Adv. Mater. 2001, 13, 1389–1393. (9) Yamamoto, M.; Kashiwagi, Y.; Sakata, T.; Mori, H.; Nakamoto, M. Chem. Mater. 2005, 17, 5391–5393. (10) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663–12676. (11) Schulz-Dobrick, M.; Sarathy, K. V.; Jansen, M. J. Am. Chem. Soc. 2005, 127, 12816–12817. (12) Ibanez, F. J.; Zamborini, F. P. ACS Nano 2008, 2, 1543–1552. (13) Aslan, K.; Perez-Luna, V. H. Langmuir 2002, 18, 6059–6065. (14) Lu, C.; Zu, Y. B.; Yam, V. W.-W. Anal. Chem. 2007, 79, 666–672. (15) Lu, C.; Zu, Y. B. Chem. Commun. 2007, 37, 3871–3873. (16) Huang, C.-C.; Tseng, W.-L. Anal. Chem. 2008, 80, 6345–6350. 10969

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