Optically Active Ultrafine Au–Ag Alloy Nanoparticles Used for

Aug 23, 2017 - Chiroptical behavior of bimetallic Au–Ag alloy NPs and the corresponding monometallic counterparts. (a, c, and e) CD spectra for the ...
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Optically Active Ultrafine Au-Ag Alloy Nanoparticles Used for Colorimetric Chiral Recognition and Circular Dichroism Sensing of Enantiomers Jianjia Wei, Yanjia Guo, Jizhou Li, Mengke Yuan, Tengfei Long, and Zhongde Liu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b01723 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 24, 2017

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Analytical Chemistry

Optically Active Ultrafine Au-Ag Alloy Nanoparticles Used for Colorimetric Chiral Recognition and Circular Dichroism Sensing of Enantiomers Jianjia Wei†, Yanjia Guo†, Jizhou Li†, Mengke Yuan†, Tengfei Long†, and Zhongde Liu∗† †

Key Laboratory of Luminescent and Real-Time Analytical Chemistry (Southwest University), Ministry of Education, College of Pharmaceutical Sciences, Southwest University, Chongqing 400716, China.

ABSTRACT: Despite a significant surge in the number of investigations into chirality at the nanoscale, especially thiolated chiral molecules capping gold clusters, only limited knowledge is currently to elaborate alloying effect on chiroptical behaviour of bimetallic nanoparticles (NPs). Also, few successful cases as to the efforts towards the development of chirality-dependent applications on the optically active nanomaterial have been made. Herein, as a positive test case for chiral alloy nanoparticle synthesis, the stable and large chiroptical ultrafine Au-Ag alloy NPs were prepared by reduction of different molar fraction of HAuCl4 and AgNO3 with NaBH4 in the presence of D/L-penicillamine (D/L-Pen). Compared with those of monometallic Au and Ag counterparts with comparable size, the Au-Ag alloy NPs (Ag mole fraction, 70%) obviously displayed the largest optical activities with the maximum g factors of ~1.6 × 10-3. Impressively, the Pen-mediated synthesis of chiral Au-Ag alloy NPs possess colorimetric self-recognition function and can be used as an incisive CD probe towards D- and L-Pen enantiomers. The plasmonic CD signal amplification (∆ICD) shows good linearity with the amount of Pen over the range of 5.0 to 80.0 µM with a detection limit (3σ) of 1.7 µM for L-Pen and 1.5 µM for D-Pen, respectively. In addition, the sensing system exhibits good selectivity towards D- and L-Pen in the presence of other enantiomers, therefore it is highly expected that the approach described here would open new opportunities for design of more novel enantioselective analysis of important species related to biological processes.

Triggered by potential applications in the fields of heterogeneous enantioselective catalysis1-3, chiral separation4-5, chiral recognition and sensing5-7, and nonlinear optics8, chirality in nanostructures has emerged as a new frontier in nanoscience. Up to now the extensive investigations have concentrated on exploration of synthesis methods and understanding the evolution of the chiroptical activity. Several typical strategies have been practiced to attain optical active nanostructures, including ( ⅰ ) the direct synthesis in the presence of chiral ligands9, 10, (ⅱ) postsynthetic modification of achiral nanoparticles (NPs)1, 11, (ⅲ) chiral assembly of NPs 12, 13 . Using such strategies, many types of chiral NPs have been reported, such as chiral Au nanospheres and nanorods, chiral Ag NPs, and even semiconductor quantum dots4, 8-12, 14-15. Besides the synthesis methods, another important focus on chirality at the nanoscale is the evolution of the chiroptical activity. For plasmonic NPs, the circular dichroism (CD) response in the vicinity of the plasmon frequency is attributed to the surface molecule induced chiral currents in the NPs 16, 17, and for nanoclusters (NCs), the formulated suggestions for the elusive mechanism can be divided into three major categories18, 19: (i) the intrinsically chiral metallic core; (ii) the dissymmetric field model; and (iii) the chiral footprint model. However, compared with the explorations of synthesis strategy and the origin of the optical activity, to obtain stable and large chiroptical response and explore

chirality-dependent applications on the optically active metallic nanostructures is currently underway. So far most of efforts towards the development of optically active nanostructures have been focused on single-component systems, especially thiolated chiral molecules capping gold clusters9, 20-30, however, compared with single-component systems, alloy metal NPs often demonstrate unique stability, leading to enhanced chemical, catalytic and optical properties31-34. Thus in this study, as a test case for chiral alloy nanoparticle synthesis, optically active ultrafine Au-Ag alloy NPs were attempted to synthesize via reduction of different molar fraction of HAuCl4 and AgNO3 by employing NaBH4 as the reducing agent and D/L-penicillamine (D/L-Pen) as the capping agent. Further, according to the report by Nishida and co-workers35, the chiroptical response of the silver NCs is several-fold larger than that of the gold NCs having the same ligand, therefore, to gain insight into the alloying effect on the chiroptical activity, the chiroptical behaviour of bimetallic Au-Ag NPs was compared with those of monometallic Au and Ag counterparts with comparable size. Optical spectroscopy shows that the electronic structures of Au-Ag alloy NPs are sensitive to Ag doping and significantly modulated by

To whom all correspondence should be addressed. Tel: 86-23-68251910; Fax: 86-23-68251048; E-mail: [email protected].

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Analytical Chemistry

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incorporation of Ag atoms. Interestingly, differing from the chiral glutathione-protected Au-Ag NCs reported by Kobayashi and co-workers34, in which a decrease in chiroptical response was observed due to Ag doping, in our study the bimetallic Au-Ag alloy NPs exhibit the stronger CD response than those of the pure Au and Ag counterparts. Such an increase in chiroptical response could be explained in terms of near-field interaction mechanism, namely the dipole and multipole interactions between a chiral molecule and a nanoparticle16. It is well-known that the enantioselective analysis of chiral molecules is a central aspect in chemical and biological research, especially in pharmaceutical science. Over the past decades, traditional methods targeted to distinguish chiral molecules have been greatly made36-38, such as chiral high-performance liquid chromatography, gas chromatography, capillary electrophoresis, but few cases have been reported on development of time- and cost-effective sensing strategies for the stereoselective analysis of the enantiomers. Thus, the development of simple, rapid, sensitive and high-throughput routine optical assay for chiral recognition, in particular, solution-based enantioselective analysis is highly desirable, and in this regard, chiroptical methods may be quite versatile. The second line of our research related to the as-prepared chiral bimetallic NPs toward exploring chirality-dependent application revealed that the Pen-mediated synthesis of chiral Au-Ag alloy NPs possess colorimetric self-recognition function for D- and L-Pen enantiomers, and the procedure can be schematically illustrated in Scheme 1, in which Au-Ag alloy nanoparticle enantiomers can be used for the colorimetric enantiodiscrimination and the CD sensing of the Pen. To the best of our knowledge, a very limited number of successful cases as to the efforts towards the development of colorimetric enantiosensing have been made5, and therefore the approach described here would provide an alternative avenue to design the more novel enantiosening strategies.

Scheme 1 Illustration of the formation of optically active bimetallic Au-Ag alloy NPs and the chirality-dependent colorimetric recognition function for the Pen enantiomer.

EXPERIMENTAL SECTION Materials and Reagents. Chloroauric acid tetrahydrate (HAuCl4·4H2O) and AgNO3 were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). The D- and L-enantiomeric forms of penicillamine (Pen) were all purchased from Sigma-Aldrich (Shanghai). Sodium borohydride (NaBH4) was purchased from Huanwei Fine Chemical Co., Ltd (Tianjin, China). All other reagents were of analytical reagent grade without further purification, and ultrapure water (18.2 MΩ) was used throughout.

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Apparatus. The UV-vis absorption spectra were recorded with a UV-3600 spectrophotometer (Shimadzu, Tokyo, Japan). A Jasco J-810 circular dichroism (CD) spectroscopy (Tokyo, Japan) was used to measure the ellipticity of the NPs. Transmission electron microscopy (TEM) images were taken with a transmission electron microscope (JEM- 1200EX). FT-IR spectra were measured with a Shimadzu 8400S (Tokyo, Japan) infrared spectrophotometer. The information about the elemental composition and the valence of Au-Ag alloy NPs was carried with an ESCALAB 250 X-ray photoelectron spectrometer (XPS) and an energy dispersive X-ray spectrometer (EDX).

Fabrication of Optically Active Nanoparticles. The bimetallic Au-Ag alloy NPs with the varying mole fractions of metal precursors are prepared using the same procedure, using predetermined initial mole ratios of the gold and silver ions in the solution. For example, 1000 µl of 25 mM HAuCl4 and 500 µl of 50 mM AgNO3 were at first mixed (Ag mole fraction, 50%), followed by the addition of 10 ml of 20 mM enantiopure Pen. Next, 2.5 ml of a freshly prepared 0.2 M NaBH4 solution all was immediately injected under vigorous stirring and the mixture was then diluted to 15 ml. After further stirring for 1.5 hour in ice bath, the solution was stored overnight. The Pen enantiomer protected monometallic counterparts were also prepared in a similar manner. Circular Dichriosm Sensing of Enantiomers. 250µl of the as-prepared Au-Ag alloy NPs and 50µl of BR buffer (pH 7.4) were at first pipetted into a 1.5-ml vial. Subsequently, an appropriate volume of Pen working solution or sample solution was added, diluted to 500µl with Milli-Q purified water and vortex-mixed thoroughly. The mixture was placed at room temperature for at least 30 min, and then transferred for CD measurements. The real samples, including both human urine and serum samples, were sampled from individuals deemed to be healthy after physical examination at the Hospital of the Southwest University. Firstly, we have spiked the known concentration of standard D-Pen at different concentration levels within the urine and serum samples by a standard addition method. The serum samples were then mixed with equal volume of acetonitrile and vortex-mixed thoroughly. Subsequently, centrifugal sedimentation was carried out for 10 min at 10000 rpm to remove the protein. After that, the top liquid was collected and heated to half of the original volume to remove the residual acetonitrile. Then the remaining solution was diluted to the original volume with purified water and used for the detection according to the general procedure. Human urine samples were filtered with 0.22 µm microporous membrane and directly used for the analysis using the proposed method. RESULTS AND DISCUSSION The Au-Ag Alloy Nanoparticle Formation. In the typical synthesis of Au-Ag alloy NPs, the change of solution color resulted from the reduction of gold and silver ions is dependent on the concentration of metal precursors, and as the mole fraction of Ag within the alloy NPs decrease, the intensity of the reddish color increase. In line with the directly observable change in appearance, absorption spectra indicate that the pure Ag NPs show a characteristic localized surface plasmon resonance (LSPR) located at ~409 nm and the pure Au NPs have a characteristic one at ~526 nm, respectively, and upon increasing the mole fraction of Au precursors, the

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Analytical Chemistry

LSPR peak is red-shifted as shown in Figure 1a. The relationship between the composition of alloy NPs and LSPR peak position was approximately linear (the inset of Figure 1a). Generally, core-shell or nonalloy Au-Ag NPs display two characteristic plasmon resonance peaks, in which one peak increases in intensity as that mole fraction increases with a concomitant decrease in intensity in the other peak39, and therefore, these data excluded the core-shell growth in the co-reduction of Au and Ag precursors. To further confirm that the alloy NPs were obtained through present co-reduction strategy, a second set of experiments were conducted by comparing the absorption spectra between the alloy particles and the mixture of pure Au and Ag NPs. The spectroscopic results indicate that when the mole ratio of the Au and Ag precursors is 3:7, a single peak can be seen at ~448 nm, conversely, in the case of the mixed pure Au and Ag particles, two separate peaks at ~526 nm and ~409 nm can be clearly observed (Figure 1b), corresponding to the LSPR frequency of the alone Au and Ag, respectively, supporting the suggestion made above that the components consist of alloy NPs.

Additionally, two fitting peaks at 367.9 eV and 373.9 eV are observed in the Ag 3d XPS spectrum (Figure 2e), which correspond to Ag 3d5/2 and Ag 3d3/2, respectively. These results suggest that both Au (0) and Ag (0) coexist in the particles and combined with the observation of uniform electron density from HRTEM, it can be concluded that the co-reduced bimetallic solution should be alloy rather than core-shell NPs. Also, the FTIR spectra were exploited to gain further insight into chemical and surface properties of the prepared alloy NPs (Figure S1). The peak at ~2523 cm-1 that corresponds to the S-H stretching vibration mode disappear in the alloy NPs, indicating that the Pen molecules may anchor on the surface of alloy NPs due to strong metal-ligand bonds43-45.

Figure 1 (a) The absorption spectra of Au-Ag alloy NPs with varying molar ratios of the Au and Ag precursors. Inset displaying the dependence of the maximum absorbance wavelength of the alloy NPs on the mole ratio of the two metal precursors; (b) the absorption spectra of a mixture of the Au and Ag NPs alone compared to 3:7 Au/Ag mole ratio alloy NPs.

More intuitive information about the morphology, size and electron density of the alloy NPs could be provided by trans-mission electron microscopy (TEM) studies. The alloy nanospheres display the well-dispersed nature and the average diameter of them is 3.3 ± 0.4 nm. According to the report by Jin and co-workers40, when the size of nanoparticle is