Nanomaterials and Nanotechnology ARTICLE

Polarimetric Detection of Enantioselective Adsorption by Chiral Au Nanoparticles – Effects of Temperature, Wavelength and Size

Invited Article

Nisha Shukla2, Nathaniel Ondeck1, Nathan Khosla1, Steven Klara1, Alexander Petti1 and Andrew Gellman1*

1 Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA, USA 2 Institute of Complex Engineered Systems, Carnegie Mellon University, Pittsburgh, PA, USA * Corresponding author(s) E-mail: [email protected]

Received 01 November 2014; Accepted 07 January 2015

DOI: 10.5772/60109

© 2015 The Author(s). Licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract 1. Introduction

R- and S-propylene oxide (PO) have been shown to interact The importance of chirality arises from the need of the enantiospecifically with the chiral surfaces of Au nanopar‐ pharmaceutical industry, and other producers of bioactive ticles (NPs) modified with D- or L-cysteine (cys). This compounds, for chemical processes that are enantioselec‐ enantiospecific interaction has been detected using optical tive and thereby produce enantiomerically pure chiral polarimetry measurements made on solutions of the D- or products. [1-4] This is one of the most challenging forms of L-cys modified Au (cys/Au) NPs during addition of chemical synthesis and is critical to the performance of racemic PO. The selective adsorption of one enantiomer of bioactive chiral products. Ingestion of the two enantiomers the PO onto the cys/Au NP surfaces results in a net rotation of a chiral compound by a living organism can result in of light during addition of the racemic PO to the solution. dramatically different physiological impacts, ranging from In order to optimize the conditions used for making these therapeutic for one enantiomer of a pharmaceutical to toxic measurements and to quantify enantiospecific adsorption for the other. Enantioselective chemical processing requires onto chiral NPs, this work has measured the effect of the use of enantiomerically pure chiral media. [5], [6] These temperature, wavelength and Au NP size on optical could be enantiomerically pure chiral reagents, chiral rotation by solutions containing D- or L-cys/Au NPs and homogeneous catalysts, chiral solvents, or chiral surfaces racemic PO. Increasing temperature, decreasing wave‐ for adsorption or heterogeneous catalysis. Needless to say, length and decreasing NP size result in larger optical chiral nanostructured materials have a role to play in rotations. enantioselective chemical processing technologies. [5], [7]

Keywords chiral, nanoparticle, enantioselective, adsorp‐ Over the past two decades, there has been enormous tion, separation, Au progress in the ability to synthesize nanoparticles (NPs)

Nanomater Nanotechnol, 2015, 5:1 | doi: 10.5772/60109 1 with exquisite control over size, shape, morphology and difference in optical rotation in the two phases yields a net function. [8-10] Most recently, there has been growing optical rotation of light. This phenomenon can be descri‐ interest in and appreciation of the fact that some metal NPs bed quite easily to yield a model that is parameterized by can be chiral. [11-17] Although metals have achiral bulk the four adsorption equilibrium constants and four specific structures, metal NPs can be chiral. The simplest form of optical rotation constants (four independent parameters in chirality induction into a metal NP occurs by adsorption of total). Given a sufficient number of measurements of optical a chiral ligand onto the NP surface. Such ligands render the rotation by solutions containing mixtures of R- and S-PO surface chiral simply by creating a chiral environment at with D- and L-cys/Au NPs, it is possible to extract quantita‐ the surface, a process often called chiral templating. tive estimates of the ratios of the adsorption equilibrium K K However, ligands can also imprint chirality onto the constants: R/D = S /L . These ratios can be used to esti‐ K R/L KS /D structure of the adsorbing NP by reconstructing its surface mate the enantiospecific difference in the free energies of such that the metal atoms adopt a chiral structure. [18], [19] adsorption of PO on cys/Au: ∆ ∆GDL = ∆GR/D - ∆G R/L Alternatively, metallic NPs can expose surfaces with high = ∆GS /L - ∆GS /D. Miller index facets, which are naturally chiral and, there‐ fore, capable of enantiospecific adsorption. [1, 2, 7, 20-26] The use of optical rotation measurements as a means of The key to giving any of these chiral NPs enantiospecific quantifying enantiospecific adsorption constants relies on properties is that they are prepared in enantiomerically careful measurements of optical rotation in solutions with pure form, in some cases simply by modifying their many different concentrations of chiral probe molecules surfaces with enantiomerically pure chiral ligands such as and chiral NPs. Optical rotation will depend on the 2, 2’-bis(diphenylphosphino)-1, 1’-binaphthyl, [27] N- wavelength of the light being used for the measurements iosobutyryl-L-cysteine, [14] or penicillamines. [28] For the and on the concentrations of probe molecule enantiomers most part, previous work on chiral NPs has focused on their in the solution phase and adsorbed phase. In turn, the structure and on their optical properties, [11], [12], [28]-[31] concentrations will depend on temperature through the however, attention has been focussed more recently on temperature dependence of the adsorption equilibrium their enantiospecific interactions with chiral probe mole‐ constants. The concentrations will depend on NP size cules. [32], [33] Ultimately, this effort will lead to enantio‐ through the net surface area for adsorption. In principle, selective NP functionality in processes such as separations the adsorption equilibrium constants could depend on NP and catalysis. size, although the size of the NPs used in this work have been quite large (4 - 80 nm) and a strong influence of NP This work furthers a methodology originally developed by size on the adsorption equilibrium constants seems the authors for the use of optical polarimetry measure‐ unlikely. The influence of wavelength, temperature and NP ments as a means of quantifying the enantiospecific size on optical rotation is complex, and so the empirical interactions of chiral probe molecules with chiral Au NPs. study described herein has been performed to explore their [32], [33] The Au NPs are rendered chiral by modification of influence on optical rotation in order to guide future work their surfaces with either D- or L-cysteine (cys). The key observation that reveals their enantioselectivity is that towards choosing experimental parameters that will during the addition of racemic propylene oxide (rac-PO) to maximize measurement sensitivity and thereby optimize solutions containing either D- or L-cys/Au NPs there is a the quality of the resulting data. change in optical rotation of linearly polarized light. If the solution contains a racemic mixture of D- and L-cys/Au NPs, 2. Experimental then the addition of rac-PO causes no rotation of light. The Au (III) chloride hydrate (HAuCl xH O, >99.9 %), trisodi‐ rotation of light by addition of rac-PO to solutions of 4 2 um citrate dihydrate (C H Na O 2H O, 99 %), sodium enantiomerically pure Au NPs arises from two effects. One 6 5 3 7 2 borohydride (NaBH , 99.9 %), L-cys (C H NO S, >97 %), D- is the fact that the enantiomers of PO adsorb enantiospecif‐ 4 3 7 2 ically to the chiral Au NPs because their adsorption cys (C3H7NO2S, 99 %) and rac-cys (C3H7NO2S, >97 %) were purchased from Sigma Aldrich and were used without equilibrium constants are enantiospecific: KR/D = KS /L further purification. R- and S-propylene oxide (C H O, 99 ≠ K = K , where K is the adsorption equilibrium 3 6 S ⁄D R⁄L R/D %) were purchased from Alfa Aesar. constant for R-PO on D-cys/Au NPs. As a result, the concentrations of the two enantiomers of PO in solution 2.1 Synthesis of chiral 4 nm Au NPs differ, and their concentrations in the adsorbed phase also differ. The second effect that induces optical rotation is that The study of enantioselective separation at different the specific optical rotation of the PO in solution differs from temperatures and using different optical wavelengths was sol sol ads ads that of PO in the adsorbed phase: -α R =αS ≠αS =-α R , conducted with cys/Au NPs with diameters of ~4 nm sol where αS is the specific optical rotation of S-PO in the synthesized as reported in earlier work. [32] A 20 mL solution phase. Thus, although the total amounts of each aqueous (distilled water) solution of 2.5 × 10−4 M Au(III) −4 enantiomer of PO added to the solution as a racemic mixture chloride hydrate (HAuCl4 xH2O) and 2.5 × 10 M trisodium

are identical, their unequal partitioning between the citrate dihydrate (C6H5Na3O7 2H2O) was prepared in a solution phase and the adsorbed phase, coupled with the three-neck round bottom flask. After 10 minutes of vigo‐

2 Nanomater Nanotechnol, 2015, 5:1 | doi: 10.5772/60109 rous mixing using a magnetic stir plate, 0.6 mL of freshly uncertainties propagated from the scatter of the data prepared 0.1 M sodium borohydride (NaBH4) was injected around the best fit line. into the flask. The solution was shielded from light and was After the 6 ml cys/Au NP solution sat for two hours allowed to stir for 24 hours or until the colour changed from following sonication, a 2 ml aliquot was injected into the a bright red to a deep red or purple. This resulted in a TempTrol Hastelloy measurement cell and placed into the solution of Au NPs. The Au NPs were modified by mixing Rudolph Autopol VI. The instrument was programmed to 1 mL of the Au NP solution with 5 mL of 0.025 M cys in wait for 10 minutes and then record 10 consecutive distilled water. The solution was sonicated for approxi‐ measurements of optical rotation and report the average of mately 40 minutes and then allowed to sit for approximate‐ these values. Immediately after the program was started, ly two hours before making optical rotation measurements. the desired amount of rac-PO was mixed with the remain‐ ing 4 ml of cys/Au NP solution and briskly shaken. After 2.2 Synthesis of 50–80 nm Au NPs the 10 minute time delay and 10 consecutive measurements of optical rotation, the 2 ml sample in the TempTrol The synthesis of variable-size Au NPs was adapted from a Hastelloy measurement cell was added to the 4 ml solution procedure published by Nasir et al. [34] The synthesis was of chiral cys/Au NPs with rac-PO and thoroughly mixed. performed in a three-neck flask covered in aluminium foil From that solution a 2 ml aliquot was removed and placed to eliminate light exposure and avoid degradation of the in the TempTrol cell to start the next set of optical rotation NPs. A 50 mL solution of (0.005 g) Au(III) chloride hydrate measurements. Additional rac-PO was then added to the (HAuCl4 xH2O) was prepared in deionized water in a round remaining 4 ml of solution, and the entire process repeated bottomed flask. The flask was heated to boiling with a reflux until the maximum desired PO concentration was reached. condenser while being continuously stirred. A 1% solu‐ The Rudolph Research Analytical Autopol VI has the tion of trisodium citrate dihydrate (C6H5Na3O7 2H2O) in ability to switch between six wavelengths and to control deionized water was quickly added to the boiling solu‐ the temperature of the sample in the TempTrol Hastelloy tion. The volume added dictated the final size of the NPs. measurement cell. This work used wavelengths of 365, 436 Adding volumes of 260 or 210 μL resulted in NPs with and 546 nm and temperatures of 23, 30 and 40 °C. diameters of 50 or 80 nm, respectively, as determined by TEM and UV-vis. Once the trisodium citrate dihydrate 3. Results and discussion (C6H5Na3O7 2H2O) was added, the colour of the synthesis mixture began to change from yellow/clear to black and then Following the procedures developed and described in our to red (for smaller particles) or purple (for larger parti‐ previous studies of optical rotation by solutions of PO and cles). The colour change occurs within a few seconds and chiral cys/Au NPs, [32], [33] this study has measured the after the colour change stopped, the solution was refluxed optical rotation of solutions prepared by adding rac-PO in for an additional 10 minutes. The solution was then removed increasing concentrations to solutions containing D- or L- from the heat source, and continuously stirred until it cys/Au NPs. The D- and L-cys/Au NP solutions (without reached room temperature. The solution was then stored in PO) rotate linearly polarized light as a result of the chirality the dark where it remained stable for several days. of the cys/Au NPs and the residual D- or L-cys in the solution phase. If there were no enantiospecific interactions 2.3 Optical rotation measurements between the enantiomers of PO and the D- or L-cys/Au NPs, All optical rotation measurements were made with a the addition of a rac-PO would not result in any change in Rudolph Research Analytical Autopol VI polarimeter optical rotation. Note that the detectable interactions using a TempTrol Hastelloy measurement cell. In order to between the enantiomers of PO and the D- or L-cys in allow the cell to reach the desired temperature, sample solution are weak with respect to those between PO and D- solutions were allowed to sit in the cell mounted in the or L-cys/Au NPs. [32] The goal of this work has been an polarimeter for 10 minutes prior to making measurements. empirical study of the effects of light wavelength, temper‐ Measurements of optical rotation were made using ature and NP size on the degree of optical rotation observed wavelengths of 365, 436 and 546 nm with temperatures of in these types of experiments. This will guide the choice of 23, 30 and 40 °C. Ten measurements of optical rotation were conditions used for future quantitative measurements of made at each of these conditions. The data reported in this the enantiospecific equilibrium constants for adsorption of paper are measurements of optical rotation angle versus chiral probe molecules on chiral NPs. concentration of R-, S-, or rac-PO and quantified as the slopes of these data in units of °/M, all of which were 3.1 Wavelength dependence of optical rotation by rac-PO in cys/ measured in the linear regime of their dependence on Au NP concentration. Although these measurements do not allow an absolute determination of the coverage of PO on the cys/ The effect of wavelength on optical rotation was observed Au NPs, the linear regime suggests that the coverages are during addition of rac-PO to solutions containing chiral low relative to the saturated monolayer. Each reported and achiral cys/Au NPs with diameters of ~4 nm. Figure 1 value of the optical rotation is the average of 10 measure‐ shows the effect of wavelength on optical rotation of light ments. The uncertainties reported for the slopes are the by solutions of D- or L-cys/Au NPs measured as a function

Nisha Shukla, Nathaniel Ondeck, Nathan Khosla, Steven Klara, Alexander Petti and Andrew Gellman: 3 Polarimetric Detection of Enantioselective Adsorption by Chiral Au Nanoparticles – Effects of Temperature, Wavelength and Size of rac-PO concentration and at 23 °C. The specific optical these measurements by a factor of 2. The measurements of rotation constants are wavelength dependent, [35], [36] and optical rotation by pure R- or S-PO in the presence of bare would influence the sensitivity of the measurements, if Au NPs and rac-cys/Au NPs (Figure 2) shows the enhance‐ there is a difference in the wavelength dependences of the ment of optical rotation that arises from adsorption of the optical rotation constants for PO in the solution phase, PO on the NPs. These show smaller enhancements of α sol, and in the adsorbed phase, α ads. The chirality of the D- optical rotation with respect to measurements at 436 nm than in the case of pure R- or S-PO but significant reduction or L-cys/Au NPs is evident from the fact that they rotate in the uncertainty of the slopes. Enantiospecific interactions light prior to the addition of rac-PO (Figure 1). The magni‐ of the R- and S-PO with D- or L-cys/Au NPs are revealed tude of optical rotation increases from 0.1° to 0.2° as the by the optical rotation measurements in Figure 3. The wavelength is decreased from 546 to 365 nm. The enantio‐ magnitudes of the optical rotations of R- and S-PO are specific interactions of the R- and S-PO with the D- or the enhanced with respect to the solution phase. More impor‐ L-cys/Au NPs are evident from the increase in the optical tantly, for S-PO the rotation is greater in the presence of D- rotation by addition of rac-PO to the solutions. The slopes cys/Au than in the presence of L-cys/Au. Consistent with a of the curves are indicated at the left side of the plot in diastereomeric relationship, the opposite is true for R-PO. Figure 1 and increase by a factor of ~3 as the wavelength is This is demonstrated more convincingly in these measure‐ decreased from 546 to 365 nm. The variance in the meas‐ ments at 365 nm than in prior work at 436 nm. Finally, ured slopes seems to increase more slowly than the Figure 4 shows the rotation of light during addition of rac- magnitude of the slope indicating that the shorter wave‐ PO to solutions containing D-cys, L-cys, rac-cys/Au, D-cys/ lengths are slightly better suited to giving higher accuracy Au or L-cys/Au NPs. The first three are control experi‐ measurements. ments. The solutions containing either D- or L-cys (0.025 M) are chiral and rotate light. However, they exhibit relatively weak enantiospecific interactions with PO; the addition of Slopes Wave- rac-PO results in small changes in the optical rotation 0.4 length relative to the rotations observed in the presence of D- or (nm) 0.23 ± 0.01 D -cys/Au L-cys/Au NPs. The rac-cys/Au NPs are not chiral, do not 0.12 ± 0.01 0.3 rotate light and, not surprisingly, the addition of rac-PO to 0.06 ± 0.005 365 the solution does not cause any rotation of light. The key 0.2 containing D-cys, L-cys, rac-cys/Au, D-cys/Au or L-cys/Au NPs. The first three are control experiments. The solutions ) 436 point is that the addition of rac-PO to the solutions con‐ containing either D- or L-cys (0.025 M) are chiral and rotate light. However, they exhibit relatively weak o tainingenantiospecific either interactions D- or with L- PO;cys/Au the addition NPs of rac-PO results results in in smalla net changes rotation in the optical rotation relative to 0.1 546 ofthe light. rotations These observed data in the obtained presence of D at- or 365L-cys/Au nm NPs. exhibit The rac-cys/Au the enhanced NPs are not chiral, do not rotate light and, not surprisingly, the addition of rac-PO to the solution does not cause any rotation of light. The key point is that opticalthe addition rotation of rac-PO toof th e~90 solutions % overcontaining that either obtained D- or L-cys/Au at NPs 436 results nm in a asnet rotation of light. These 0.0 showndata obtained in Figure at 365 nm 1.exhibit the enhanced optical rotation of ~90 % over that obtained at 436 nm as shown in Figure 1.

-0.1 546 1.6 Slopes rac-cys/Au -0.2 436

Optical Rotation ( 2.02 ± 0.01 1.2 bare Au 1.63 ± 0.03 S -PO -0.3 -0.07 ± 0.01 1.42 ± 0.04 365 ) 0.8 -0.12 ± 0.02 o L-cys/Au -0.22 ± 0.02 -0.4 0.4 S-PO 0.0 0.1 0.2 0.3 0.4 [rac-PO] (M) 0.0

Figure 1. Optical rotation of linearly polarized light at wavelengths of λ = -0.4 R-PO 546,Figure 436 1. Opticaland 365 rotation nm of during linearly polarizedaddition light of at rac-PO wavelengths to solutionsof λ = 546, 436 containing and 365 nm during D- oraddition of rac-PO to L-solutionscys/Au containing NPs. Quantities D- or L-cys/Au at NPs. left Quantities are the at slopesleft are the of slopes rotation of rotation angle angle versus versus PO PO concentration. Measurements -5 Optical Rotation ( -0.8 concentration.made at: T = 23 °C, Measurements NP diameter ~4 nm, [Au]made = 1.6×10 at: T g/mL, = 23 [cys] °C, = NP0.025 diameterM, pH = 8. ~4 nm, [Au] -1.49 ± 0.04 -1.61 ± 0.02 R-PO = 1.6×10A set of -5measurements g/mL, [cys] of = optical 0.025 rotation M, pH by = R-8., S-, or rac-PO in solution with D-cys/Au, L-cys/Au, rac-cys/Au or bare Au NPs have been made using 365 nm light and a temperature of 23 °C. These are comparable to experiments-1.2 -1.99 ± 0.01 bare Au reported earlier using 436 nm light and room temperature, without temperature control. [32] As shown in Figure 2, rac-cys/Au Athe set specific of measurements rotation of R- and S-PO of at 365optical nm has arotation value of by = 1.45R- ,°/M S- which, or rac-is about 70 % higher than that POmeasured in solution previously withusing 465 D- nmcys/Au, light. Equally L- cys/Au,important, the rac-cys/Au temperature cont orrol barereduces the uncertainty in these-1.6 measurements by a factor of 2. The measurements of optical rotation by pure R- or S-PO in the presence of bare Au 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 AuNPs NPs and rac-cys/Au have been NPs (Figure made 2) shows using the enhancement 365 nm oflight optical and rotation a temper‐that arises from adsorption of the PO atureon the NPs. of These 23 show °C. smaller These enhancem areents comparable of optical rotation with to respect experiments to measurements at 436 nm than in the [PO] (M) case of pure R- or S-PO but significant reduction in the uncertainty of the slopes. Enantiospecific interactions of the R- reportedand S-PO with earlier D- or L- cys/Auusing NPs 436 are revealednm light by the and optical room rotation temperature,measurements in Figure 3. The magnitudes of Figure 2. Rotation of light versus [PO] by enantiomerically pure R- and S- withoutthe optical rotationstemperature of R- and S- control.PO are enhanced [32 ]with As respect shown to the in so lutionFigure phase. 2, More the importantly, for S-PO the rotation is greater in the presence of D-cys/Au than in the presence of L-cys/Au. Consistent with POa diastereomericFigure in aqueous 2. Rotation solutionof light versus with [PO] andby enantiomerically without bare pure AuR- and and S-PO rac-Au in aqueous NPs. solution In waterwith and without bare Au and rac- specificrelationship, rotation the opposite of is true R- for and R-PO. S- ThisPO is demonstrated at 365 nm more has convinci a valuengly in these of measurements(●)Au the NPs. at R-365 In and waternm S-(●)PO the R-rotate and S- POlight rotate by light equal by equal and and opposite opposite angles angles with awith specific a specificrotation of ~1.4 °/M. In the presence than in prior work at 436 nm. Finally, Figure 4 shows the rotation of light during addition of rac-POof tobare solutions Au NPs ( ■) and rac-cys/Au NPs (○) the specific rotation is measurably increased. Measurements made at: λ = 365 nm, T = 23 |α sol| = 1.45 °/M which is about 70 % higher than that rotation°C, NP diameter of ~1.4 ~4 °/M. nm, [Au] In =1.6×10the presence-5 g/mL, [cys] of = bare0.025 M, Au pH NPs= 8. (■) and rac-cys/Au NPs (○) the specific rotation is measurably increased. Measurements made at: measured previously using 465 nm light. Equally impor‐ -5 λ = 365 nm, T = 23 °C, NP diameter ~4 nm, [Au] =1.6×10 g/mL, [cys] = 0.025 tant, the temperature control reduces the uncertainty in M, pH = 8.

4 Nanomater Nanotechnol, 2015, 5:1 | doi: 10.5772/60109

Temperature influences the system through the enantio‐ 1.6 D -cys/Au specific adsorption equilibrium constants, KR/D (and

1.2 2.07 ± 0.03 L-cys/Au others). In principle, measuring the ratios of the adsorption K 1.64 ± 0.03 S-PO equilibrium constants, R/D , over a range of temperatures 1.43 ± 0.04 KS /D 0.8 ) allows direct determination of the enantiospecific heat and o

0.4 S-PO entropy of adsorption, ∆ ∆ H R-S and ∆ ∆SR-S , respectively; although the limited temperature range may make this 0.0 difficult. Figure 5 shows measurements of optical rotation by rac-PO in D-cys/Au NPs at 23, 30 and 40 °C and under -0.4 R-PO temperature control. The size of the cys/Au NPs was 4 nm and the solution pH = 8. In our previous work [32, 33] all -0.8 Optical Rotation ( -1.49 ± 0.04 experiments were performed at room temperature without -1.78 ± 0.06 R-PO -2.28 ± 0.03 temperature control. The optical rotation by the D-cys/Au -1.2 D -cys/Au NPs alone shows slight but not significant temperature L-cys/Au -1.6 dependence; however, the slopes of the optical rotation 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 with respect to rac-PO concentration double over the [PO] (M) temperature range of 23 to 40 °C. The origin of this effect is the temperature dependence of the ratios of the enantio‐ Figure 3. Optical rotation versus R- and S-PO concentration in aqueous K R/D solutionsFigure 3. Optical (●) and rotation in versussolutions R- and containingS-PO concentration D- inor aqueous L-cys/Au solutions NPs. (●) and The in datasolutions from containing D- specificor L-cys/Au adsorption equilibrium constants, . KS /D theNPs. solutions The data from containing the solutions the containing heterochiral the heterochiral pairs pairs (R- (R-PO/PO/L-cys/Aucys/Au and andS-PO/ D-S-cys/Au)PO/D- are marked with solid squares (■). The data from solutions containing the homochiral pairs (R-PO/D-cys/Au and S-PO/L-cys/Au) are marked with solid cys/Au)triangles ( are▲). The marked differences withbetween solidthe homochiral squares and the (■). heterochiral The pairs data are fromindicative solutions of the enantiospecific interaction of containingR- and S-PO withthe thehomochiral chiral Au NPs. pairsMeasurements (R-PO/ madeD- at:cys/Au λ = 365 nm, and T = 23S- °C,PO/ NPL- diametercys/Au) ~4 nm, are [Au] = 1.6×10-5 g/mL, [cys] = marked0.025 M, pHwith = 8. solid triangles (▲). The differences between the homochiral -0.04 and the heterochiral pairs are indicative of the enantiospecific interaction of R- and S-PO with the chiral Au NPs. Measurements made at: λ = 365 nm, T -0.05 -5 = 23 °C, NP diameter ~4 nm, [Au] = 1.6×10 g/mL, [cys] = 0.025 M, pH = 8. -0.06

0.3 ) -0.07 o D -cys/Au 0.21 ± 0.01 -0.08 0.2 -0.09 o 23 C ) o ( 0.1 -0.10 0.046 ± 0.003 D -cys -0.13 ± 0.01 -0.11 -0.17 ± 0.01 30oC 0.0 rac-cys/Au -0.21 ± 0.002 -0.12 Optical rotation ( rotation Optical L-cys -0.025 ± 0.003 -0.1 -0.13 40oC

Optical Rotation -0.14 -0.2 -0.21 ± 0.01 -0.15 L-cys/Au 0.0 0.1 0.2 0.3 0.4 -0.3 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 [rac-PO] (M) [rac-PO] (M) Figure 5. Optical rotation versus concentration of rac-PO in solutions containing D-cys/Au NPs at temperatures of 23, 30 and 40 °C. FigureSlopes 5. of Opticalthe data are given rotation in the left versusside of the graph. concentration Measurements made of at: rac-PO λ = 436 nm, in NP solutionsdiameter ~4 nm, [Au] = 1.6×10-5 g/mL, [cys] = 0.025 M, pH = 8. containing D-cys/Au NPs at temperatures of 23, 30 and 40 °C. Slopes of the Figure 4. Rotation of polarized light versus rac-PO concentration in data Aare set givenof measurements in the leftof optical side rotation of the by graph. R-, S-, or Measurementsrac-PO in solution with made D-cys/Au, at: λL- cys/Au,= 436 rac-cys/Au or bare solutionsFigure 4. containing:Rotation of polarized rac-cys/Au light versus NPsrac-PO (■), concentration L- or D- in solutionscys ( , containing:▲), and rac-cys/AuL- or D- cys/NPs (■), L- or D-cys (↔Au, ▲ NPs), have been made at 40 °C and a wavelength-5 of 436 nm (Figures 6–8). These are comparable to experiments and L- or D-cys/Au NPs (●, ○). The control measurements show that there are no enantiospecific interactions betweennm, PO NPand L- diametercys ~4 nm, [Au] = 1.6×10 g/mL, [cys] = 0.025 M, pH = 8. reported earlier using 436 nm light and room temperature, without temperature control (Table 1). [32] The Au (NPs↔), D- (●,cys ( ▲○).) or rac-cys/AuThe control NPs (■ measurements). PO does interact enantiospecifically show that therewith L- andare D- nocys/Au enantio‐ NPs (●, ○), thus causing increased magnitude of the specific optical rotation by R- and S-PO in solution is = 0.80 °/M, which is ~10 % lower than rotation of polarized light with increasing [rac-PO]. Measurements made at:⋆ λ = 365 nm, T = 23 °C, NP diameter ~4 nm, [Au] = specific interactions between PO and L-cys ( ), D-cys (▲) or rac-cys/Au NPs that reported earlier, and is not surprising as the optical rotation constants themselves should not be very 1.6×10-5 g/mL, [cys] = 0.025 M, pH = 8. A set of measurements of optical rotation by R-, S-, or rac- (■). PO does interact enantiospecifically with L- and D-cys/Au NPs (●, ○), temperature dependent in molecules that have only one conformation. [35] The measurements in solutions thus causing increased rotation of polarized⋆ light with increasing [rac-PO]. PO incontaining solution bare Au with or rac-cys/Au D-cys/Au, NPs (Figure L- 6)cys/Au, reveal the rac-cys/Auenhancement of optical or barerotation that arises from 3.2. Temperature dependence of optical rotation by rac-PO in cys/Au NPs adsorption on the Au NPs. However, the magnitudes of the slopes are now ~50 % lower than the values reported at Measurements made at: λ = 365 nm, T = 23 °C, NP diameter ~4 nm, [Au] = Au NPsroom temperature. have been [32] This made suggests atthat 40the increased°C and temperature a wavelength has resulted in ofa decrease 436 in the adsorption 1.6×10The-5 next g/mL, experiment [cys] =studied 0.025 theM, effect pH =of 8. temperature on enantioselective partitioning of rac-PO onnm cys/Au equilibrium (Figures NPs. constants, 6–8). consistent These with an are exothermic comparable heat of adsorption. to As experimentsin Figure 3, the data in Figure 7 showing Temperature influences the system through the enantiospecific adsorption equilibrium constants, / (andthe others). rotation of light for R- or S-PO in solution with either D- or L-cys/Au NPs reveal a diastereomeric relationship In principle, measuring the ratios of the adsorption equilibrium constants, ⁄, over a range of temperaturesreported allows earlier using 436 nm light and room temperature, ⁄ direct determination of the enantiospecific heat and entropy of adsorption, ∆∆ and ∆∆without, respectively; temperature control (Table 1). [32] The magnitude 3.2although Temperature the limited temperaturedependence range ofmay optical make this rotation difficult. Figure by rac-PO5 shows measuremen in cys/ts of optical rotation by Aurac-PO NPs in D-cys/Au NPs at 23, 30 and 40 °C and under temperature control. The size of the cys/Au NPsof was the 4 nm specific and optical rotation by R- and S-PO in solution is the solution pH = 8. In our previous work [32, 33] all experiments were performed at room temperature| so lwithout| temperature control. The optical rotation by the D-cys/Au NPs alone shows slight but not significant αtemperature = 0.80 °/M, which is ~10 % lower than that reported Thedependence; next experiment however, the slopes studied of the optical the rotation effect with of respect temperature to rac-PO concentration on doubleearlier, over the and is not surprising as the optical rotation con‐ temperature range of 23 to 40 °C. The origin of this effect is the temperature dependence of the ratios of the enantioselectiveenantiospecific adsorption partitioning equilibrium constants, of ⁄ rac-PO. on cys/Au NPs. stants themselves should not be very temperature depend‐ ⁄

Nisha Shukla, Nathaniel Ondeck, Nathan Khosla, Steven Klara, Alexander Petti and Andrew Gellman: 5 Polarimetric Detection of Enantioselective Adsorption by Chiral Au Nanoparticles – Effects of Temperature,

Wavelength and Size

ent in molecules that have only one conformation. [35] The 1.0 measurements in solutions containing bare Au or rac- D -cys/Au cys/Au NPs (Figure 6) reveal the enhancement of optical 0.8 1.20 ± 0.02 rotation that arises from adsorption on the Au NPs. 0.85 ± 0.06 L-cys/Au However, the magnitudes of the slopes are now ~50 % 0.6 0.79 ± 0.03

) S-PO

lower than the values reported at room temperature. [32] o 0.4 This suggests that the increased temperature has resulted in a decrease in the adsorption equilibrium constants, 0.2 S-PO consistent with an exothermic heat of adsorption. As in 0.0 Figure 3, the data in Figure 7 showing the rotation of light for R- or S-PO in solution with either D- or L-cys/Au NPs -0.2 R-PO reveal a diastereomeric relationship between the four -0.4 measurements, and clearly reveal an enantiospecific R-PO Optical Rotation ( Rotation Optical -0.81 ± 0.02 interaction of PO with cys/Au NPs. Finally, the addition of -0.6 D -cys/Au rac-PO to solutions containing D-cys, L-cys, rac-cys/Au, D- -0.94 ± 0.01 -0.8 -1.49 ± 0.05 cys/Au or L-cys/Au NPs induces the optical rotation shown L-cys/Au in Figure 8. The control experiments using D-cys, L-cys, and -1.0 rac-cys/Au showed no net change in optical rotation as a 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 result of adding the rac-PO. On the other hand, the net [PO] (M) rotation induced by addition of rac-PO to solutions containing the D- or L-cys/Au NPs clearly reveals enantio‐ Figure 7. Optical rotation versus concentration of R- and S-PO in aqueous specific adsorption on the NPs. The optical rotation solutionFigure 7. Optical(●) and rotation in solutions versus concentration containing of R- and D- S- POor inL- aqueouscys/Au solution NPs. ( ●The) and data in solutions from containing D- or L-cys/Au theNPs. solutions The data from containing the solutions the containing heterochiral the heterochiral pairs pairs (R- (PO/R-PO/L-L-cys/Au and and S-PO/ S-D-PO/cys/Au)D- are marked with solid measured of 0.23 °/M at 40 °C is significantly higher than squares (■). The data from solutions containing the homochiral pairs (R-PO/D-cys/Au and S-PO/L-cys/Au) are marked with solid cys/Au)triangles (▲ are). The marked differences withof the homochiral solid squares and the heterochiral (■). The pairs are data indicative from of the solutions enantiospecific interaction of R- and the value of 0.15 °/M reported earlier from measurements containingS-PO with the the chiral homochiral Au NPs. Measurements pairs (madeR-PO/ at: λD- = 436cys/Au nm, T = and40 °C, S-NPPO/ diameterL-cys/Au) ~4 nm, [Au] are = 1.6×10-5 g/mL, [cys] = 0.025 made at room temperature. [32] markedM, pH = 8.with solid triangles (▲). The differences of the homochiral and the between the four measurements, and clearly reveal an enantiospecific interaction of PO with cys/Au NPs.heterochiral Finally, the pairs are indicative of the enantiospecific interaction of R- and addition of rac-PO to solutions containing D-cys, L-cys, rac-cys/Au, D-cys/Au or L-cys/Au NPs inducesS-PO the withoptical the chiral Au NPs. Measurements made at: λ = 436 nm, T = 40 °C, rotation shown in Figure 8. The control experiments using D-cys, L-cys, and rac-cys/Au showed no net change in -5 NP diameter ~4 nm, [Au] = 1.6×10 g/mL, [cys] = 0.025 M, pH = 8. Measurementoptical rotation as conditionsa result of adding the rac-PO.rac-PO On the on other hand, the netrac-PO rotation on induced by addition of rac-PO to solutions containing the D- or L-cys/Au NPs clearly reveals enantiospecific adsorption on the NPs. The optical Temp (°C) λ (nm) L-cys/Au D-cys/Au rotation measured of 0.23 °/M at 40 °C is significantly higher than the value of 0.15 °/M reported earlier from measurements23 made at 436room temperature. [32]-0.15±0.01 0.15±0.01 0.3 23 Measurement365 conditions -0.21±0.01 rac-PO on 0.22±0.01 rac-PO on D -cys/Au Temp (°C) λ (nm) L-cys/Au D-cys/Au 40 23 365 436 -0.20±0.01 -0.15±0.01 0.25±0.01 0.15±0.01 23 365 -0.21±0.01 0.22±0.01 0.2 0.25 ± 0.01 Table 1. Slope of optical40 rotations365 versus rac-PO concentration -0.20±0.01 for solutions 0.25±0.01 withTable L- 1.or Slope D-cys/Au of optical rotationsNPs at versus different rac-PO concentration wavelengths for solutions and temperatureswith L- or D-cys/Au NPs at different wavelengths and temperatures ) o 0.1 0.011 ± 0.002 D -cys 0.8 rac-cys/Au -0.01 ± 0.01 1.05 ± 0.01 0.0 rac-cys/Au 0.6 bare Au 0.84 ± 0.02 -0.01 ± 0.01 S-PO 0.79 ± 0.03 L-cys ) 0.4 -0.1 o

S-PO ( Rotation Optical 0.2 -0.20 ± 0.01 -0.2 L-cys/Au 0.0

-0.3 -0.2 R-PO 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Optical Rotation ( Rotation Optical -0.4 [rac-PO] (M) -0.81 ± 0.03 R-PO -0.6 -0.81 ± 0.03 bare Au Figure 8. Rotation of polarized light versus concentration of rac-PO added -1.12 ± 0.01 to Figuresolutions 8. Rotation containing: of polarized light rac-cys/Au versus concentration NPs (○), of rac-PO L- or added D-cys to solutions (▲, ▼), containing: or D- orrac-cys/Au L- NPs (○), L- or D-cys rac-cys/Au cys/Au(▲, ▼), or NPs D- or L- (cys/Au, ●). NPs The (↔ , ● control). The control measurements measurements show show that there that are no there enantiospecific are no interactions between PO -0.8 and L-cys, D-cys or rac-cys/Au NPs. PO interacts enantiospecifically with D- or L-cys/Au NPs, thus causing increased rotation of 0.00.10.20.30.40.50.60.70.8 enantiospecificpolarized light with interactions increasing [rac-PO]. between Measurements PO and made L- at:cys, λ = 436 D- nm,cys T or = 40 rac-cys/Au °C, NP diameter NPs. ~4 nm, [Au] = 1.6×10-5 g/mL, PO[cys] interacts = 0.025 M, pH enantiospecifically⋆ = 8. with D- or L-cys/Au NPs, thus causing [PO] (M) increased rotation of polarized light with increasing [rac-PO]. Measure‐ ments3.3. NP made size at: effects λ = 436 on nm, optical T = rotation40 °C, NP measurements diameter ~4 nm, [Au] = 1.6×10-5 g/

Figure 6. Rotation of light versus concentration of enantiomerically pure R- mL,The [cys] optical = 0.025rotation M, measurements pH = 8. used in this study can be influenced by NP size via three mechanisms. One is or S-FigurePO 6.in Rotation aqueous of light solution, versus concentration with bareof enantiomerically Au NPs pureand R- with or S-PO rac-cys/Au in aqueous solution, NPs. with bare Au NPssimply and awith reduction in the surface area for PO adsorption. The other is an NP size effect on the specific optical rotation rac-cys/Au NPs. When dissolved in water (●) the R- and S-PO rotate light by equal and opposite angles with a specific ofrotation light ofby adsorbed PO. The third is the size dependence of the enantiospecific adsorption equilibrium constants. When1.1 °/M. dissolved In the presence in of water bare Au (●)NPs (■ the) and R- rac-cys/Au and S- NPsPO the rotatespecific rotation light isby increased equal measurably. and Measurements made The study of NP size effects has been performed by adding rac-PO to solutions of D- or L-cys/Au NPs with sizes of oppositeat: λ = 436 angles nm, T = 40with °C, NP a diameter specific ~4 nm,rotation [Au] = 1.6×10 of -51.1 g/mL, °/M. [cys] In = 0.025 the M, presence pH = 8. of bare 3.3 NP size effects on optical rotation measurements ~4, ~50, or ~80 nm. Measurements were made at 23 °C using 436 nm light. The increase in NP diameter results in a Au NPs (■) and rac-cys/Au NPs the specific rotation is increased measura‐ 400-fold decrease in the adsorbing surface area. The data in Figure 9 certainly reveal a decrease in the net rotation of bly. Measurements made at: λ = 436 nm, T = 40 °C, NP diameter ~4 nm, [Au] Thelight opticalby addition rotationof rac-PO to solutionsmeasurements containing NPs ofused different in diameter; this study however, can the net optical rotation is far -5 = 1.6×10 g/mL, [cys] = 0.025 M, pH = 8. befrom influenced linearly proportional by to NP the surfac sizee area via of threethe NPs. For mechanisms. the larger particles Onewith mean is diameters of 80 and 50 nm, the surface area increases by ~2.5 for the smaller particles which is roughly consistent with the increase in the slopes of the optical rotations. However, the increase in the slopes of the optical rotation when going from 50 to 4 nm is only an additional 50 %. Recently, we have developed a measurement and analysis protocol that allows 6 Nanomater Nanotechnol, 2015, 5:1 | doi: 10.5772/60109 measurement of the ratios of the enantiospecific adsorption equilibrium constants without the need for direct determination of the NP surface area. [33] The interesting implication of the data in Figure 9 is that the optical

simply a reduction in the surface area for PO adsorption. Increasing temperature while decreasing wavelength and The other is an NP size effect on the specific optical rotation NP size will enhance the sensitivity of these types of of light by adsorbed PO. The third is the size dependence measurement. In the system being used for these studies, a of the enantiospecific adsorption equilibrium constants. wavelength of 365 nm and temperature of 40 °C will The study of NP size effects has been performed by adding provide a ~50 % increase in optical rotation during addition rac-PO to solutions of D- or L-cys/Au NPs with sizes of ~4, of rac-PO to solutions containing chiral Au NPs. However, ~50, or ~80 nm. Measurements were made at 23 °C using from the point of view of improved measurement accuracy, 436 nm light. The increase in NP diameter results in a 400- the ability to control temperature during extended sets of fold decrease in the adsorbing surface area. The data in measurements appears to be equally important to improv‐ Figure 9 certainly reveal a decrease in the net rotation of ing measurement accuracy. light by addition of rac-PO to solutions containing NPs of different diameter; however, the net optical rotation is far 5. Acknowledgements from linearly proportional to the surface area of the NPs. This work has been supported by the DOE through grant For the larger particles with mean diameters of 80 and 50 number DE-FG02-12ER16330. NS would like to acknowl‐ nm, the surface area increases by ~2.5 for the smaller edge support from the Pennsylvania Infrastructure Tech‐ particles which is roughly consistent with the increase in nology Alliance. the slopes of the optical rotations. However, the increase in the slopes of the optical rotation when going from 50 to 4 nm is only an additional 50 %. Recently, we have developed 6. References a measurement and analysis protocol that allows measure‐ [1] Gellman A J (2010) Chiral surfaces: Accomplish‐ ment of the ratios of the enantiospecific adsorption equili‐ ments and challenges. ACS Nano 4: 5-10. brium constants without the need for direct determination [2] Sholl D S and Gellman A J (2009) Developing chiral of the NP surface area. [33] The interesting implication of surfaces for enantioselective chemical processing. the data in Figure 9 is that the optical rotation measure‐ AIChE Journal 55: 2484-2490. ments have the sensitivity to allow study of enantiospecific adsorptionrotation measurements across have a widethe sensitivity range to allow of studyNP ofsizes. enantiospecific adsorption across a wide range[3] of NPStinson S C (2001) Chiral pharmaceuticals. Chemi‐ sizes. cal and Engineering News 79 (40): 79–97 [4] Thayer A M (2007) Centering on chirality. Chemical Slope NP size and Engineering News 85: 11-19. 0.3 D -cys/Au (nm) 0.15 ± 0.01 [5] Chang C L, Wang X, Bai Y and Liu H W (2012) 0.09 ± 0.005 ~4 0.03 ± 0.003 Applications of nanomaterials in enantioseparation 0.2 and related techniques. Trends Anal. Chem. 39: )

o ~50 195-206. 0.1 ~80 [6] Mallat T, Orglmeister E and Baiker A (2007) Asymmetric catalysis at chiral metal surfaces. 0.0 Chemical Reviews 107: 4863-4890. [7] Gellman A J and Shukla N (2009) Nanocatalysis - ~80 -0.1 More than speed. Nature Materials 8: 87-88.

Optical Rotation ( ~50 [8] Cuenya B R (2010) Synthesis and catalytic proper‐ -0.2 -0.05 ± 0.003 -0.09 ± 0.004 ties of metal nanoparticles: Size shape support ~4 -0.15 ± 0.01 L-cys/Au composition and oxidation state effects. Thin Solid -0.3 Films 518: 3127-3150. 0.0 0.2 0.4 0.6 0.8 1.0 [9] Grzelczak M, Perez-Juste J, Mulvaney P and Liz- [rac-PO] (M) Marzan L M (2008) Shape control in gold nanopar‐

ticle synthesis. Chemical Society Reviews 37: Figure 9. Rotation of polarized light versus concentration of rac-PO to solutionsFigure 9. containing Rotation of polarized D- or light L- versuscys/Au concentration NPs with of rac-PO diameters to solutions ofcontaining ~4, ~50, D- or ~80 L-cys/Au nm. NPs with diameters of ~4,1783-1791. ~50, ~80 nm. The slopes of each data sets are at the left. Measurements made at: λ = 436 nm, T = 23 °C, [Au ~4 nm] = 1.6×10-5 g/mL, The [Auslopes ~50 nm of and each ~80 nm] data = 1.4×10 sets-5 g/mL, are [cys] at the= 0.025 left. M, pH Measurements = 7. made at: λ = 436 [10] Somorjai G A and Park J Y (2008) Colloid science of nm, T = 23 °C, [Au ~4 nm] = 1.6×10-5 g/mL, [Au ~50 nm and ~80 nm] = -5 metal nanoparticle catalysts in 2D and 3D struc‐ 1.4×104. Conclusions g/mL, [cys] = 0.025 M, pH = 7. tures. Challenges of nucleation growth composition The systematic study reported herein has demonstrated that optical wavelength, temperature and NP size influenceparticle shape size control and their influence on 4. Conclusionsthe magnitudes of optical rotation measurements used for determination of the enantiospecific adsorption equilibrium constants for chiral probe molecules on chiral Au NPs. Increasing temperature while decreasingactivity and selectivity. Topics in Catalysis 49: wavelength and NP size will enhance the sensitivity of these types of measurement. In the system being used for these studies, a wavelength of 365 nm and temperature of 40 °C will provide a ~50 % increase in optical rotation126-135. Theduring systematic addition of rac-PO study to solution reporteds containing herein chiral Au NPs. has However, demonstrated from the point of view of improved thatmeasurement optical wavelength,accuracy, the ability temperatureto control temperature and during NP extended size influencesets of measurements appears[11] to beLi T H, Park H G, Lee H S and Choi S H (2004) equally important to improving measurement accuracy. the magnitudes of optical rotation measurements used for Circular dichroism study of chiral biomolecules determination of the enantiospecific adsorption equilibri‐ conjugated nanoparticles. Nanotechnology 15: um constants for chiral probe molecules on chiral Au NPs. S660-S663.

Nisha Shukla, Nathaniel Ondeck, Nathan Khosla, Steven Klara, Alexander Petti and Andrew Gellman: 7 Polarimetric Detection of Enantioselective Adsorption by Chiral Au Nanoparticles – Effects of Temperature, Wavelength and Size [12] Canfield B K et al (2006) Linear and nonlinear [25] Horvath J D and Gellman A J (2002) Enantiospecific optical properties of gold nanoparticles with broken desorption of chiral compounds from chiral Cu(643) symmetry. Journal of Nonlinear Optical Physics and achiral Cu(111) surfaces. Journal of the Ameri‐ and Materials 15: 43-53. can Chemical Society 124: 2384-2392. [13] Gautier C et al (2006) Probing chiral nanoparticles [26] Horvath J D, Koritnik A, Kamakoti P, Sholl D S and and surfaces by infrared spectroscopy. Chimia 60: Gellman A J (2004) Enantioselective separation on a A777-A782. naturally chiral surface. Journal of the American [14] Gautier C and Burgi T (2006) Chiral N-isobutyryl- Chemical Society 12:6 14988-14994. cysteine protected gold nanoparticles: Preparation [27] Tamura M and Fujihara H (2003) Chiral bisphos‐ size selection and optical activity in the UV-vis and phine BINAP-stabilized gold and palladium infrared. Journal of the American Chemical Society nanoparticles with small size and their palladium 128: 11079-11087. nanoparticle-catalyzed asymmetric reaction. Journal of the American Chemical Society 125: [15] Shemer G et al (2006) Chirality of silver nanoparti‐ 15742-15743. cles synthesized on DNA. Journal of the American [28] Yao H, Miki K, Nishida N, Sasaki A and Kimura K Chemical Society 128: 11006-11007. (2005) Large optical activity of gold nanocluster [16] Gautier C and Buergi T (2008) Chiral metal surfaces enantiomers induced by a pair of optically active and nanoparticles. Chimia 62: 465-470. penicillamines. Journal of the American Chemical [17] Gautier C and Burgi T (2009) Chiral gold nanopar‐ Society 127: 15536-15543. ticles. Chemphyschem 10: 483-492. [29] Yao H (2008) Optically active gold nanoclusters. [18] Wu Z W, Gayathri C, Gil R R and Jin R C (2009) Curr. Nanosci. 4: 92-97. Probing the structure and charge state of gluta‐ [30] Hidalgo F, Sanchez-Castillo A, Garzon I L and thione-capped Au-25(SG)(18) Clusters by NMR and Noguez C (2009) First-principles calculations of mass spectrometry. Journal of the American circular dichroism of ligand-protected gold nano‐ Chemical Society 131: 6535-6542. particles. European Physical Journal D 52: 179-182. [19] Jadzinsky P D, Calero G, Ackerson C J, Bushnell D [31] Noguez C and Garzon I L (2009) Optically active A and Kornberg R D (2007) Structure of a thiol metal nanoparticles. Chemical Society Reviews 38: monolayer-protected gold nanoparticle at 1.1 757-771. angstrom resolution. Science 318: 430-433. [32] Shukla N, Bartel M A and Gellman A J (2010) [20] McFadden C F, Cremer P S and Gellman A J (1996) Enantioselective separation on chiral Au nanopar‐ Adsorption of chiral alcohols on "chiral" metal ticles. Journal of the American Chemical Society surfaces. Langmuir 12: 2483-2487. 132: 8575-8580. [33] Shukla N, Ondeck N and Gellman A J (2014) [21] Quan Z, Wang Y and Fang J (2013) High-index Quantitation of enantiospecific adsorption on chiral faceted noble metal nanocrystals. Accounts of nanoparticles from optical rotation. Surface Science Chemical Research 46: 191-202. 629: 15-19. [22] Yun Y, Wei D, Sholl D S and Gellman A J (2014) [34] Nasir S M and Nur H (2008) Gold nanoparticles Equilibrium adsorption of D- and L-alanine mix‐ embedded on the surfaces of polyvinyl alcohol tures on naturally chiral Cu{3 1 17}RandS surfaces. layer. Malaysian Journal of Fundamental Science 4: Journal of Physical Chemistry C 118: 14957-14966. 245-252. [23] Yun Y, Gellman AJ (2013) Enantioselective Separa‐ [35] Wiberg K B, Wang Y G, Murphy M J and Vaccaro P tion on Naturally Chiral Metal Surfaces: d,l- H (2004) Temperature dependence of optical Aspartic Acid on Cu(3,1,17)(R&S) Surfaces. Angew. rotation: Alpha-pinene beta-pinene pinane cam‐ Chem. Int. Edit. 52: 3394-3397. phene camphor and fenchone. Journal of Physical [24] Horvath J D and Gellman A J (2001) Enantiospecific Chemistry A 108: 5559-5563. desorption of R- and S-propylene oxide from a [36] Ioan S, Cosutchi A I and Dorohoi D-O (2008) Optical chiral Cu(643) surface. Journal of the American rotatory dispersion for polymers. Romanian Chemical Society 123: 7953-7954. Journal of Physics 53: 85-90.

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