Chiral Separation of Rac-Propylene Oxide on Penicillamine Coated Gold Nps

nanomaterials

Article Chiral Separation of rac-Propylene Oxide on Penicillamine Coated Gold NPs

Nisha Shukla 1,2, Zachary Blonder 2 and Andrew J. Gellman 2,3,*

1 Institute of Complex Engineered Systems, Carnegie Mellon University, Pittsburgh, PA 15213, USA; [email protected] 2 Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA; [email protected] 3 W.E. Scott Institute for Energy Innovation, Carnegie Mellon University, Pittsburgh, PA 15213, USA * Correspondence: [email protected]

Received: 16 July 2020; Accepted: 20 August 2020; Published: 30 August 2020

Abstract: The surfaces of chemically synthesized spherical gold NPs (Au-NPs) have been modified using chiral L- or D-penicillamine (Pen) in order to impart enantioselective adsorption properties. These chiral Au-NPs have been used to demonstrate enantioselective adsorption of racemic propylene oxide (PO) from aqueous solution. In the past we have studied enantioselective adsorption of racemic PO on L- or D-cysteine (Cys)-coated Au-NPs. This prior work suggested that adsorption of PO on Cys-coated Au-NPs equilibrates within an hour. In this work, we have studied the effect of time on the enantioselective adsorption of racemic PO from solution onto chiral Pen/Au-NPs. Enantioselective adsorption of PO on chiral Pen/Au-NPs is time-dependent but reaches a steady state after ~18 h at room temperature. More importantly, L- or D-Pen/Au-NPs are shown to adsorb R- or S-PO enantiospecifically and to separate the two PO enantiomers from racemic mixtures of RS-PO.

Keywords: chiral; nanoparticles; enantioselective adsorption; penicillamine; cysteine

1. Introduction Chirality has attracted enormous interest in the field of chemistry due to the enantiospecific responses of living organisms to the two enantiomers of chiral compounds that they have ingested. These differences in response are important in pharmaceutical industries because it is often the case that only one enantiomer of a chiral drug is therapeutically active. While one enantiomer of a drug can treat disease, the other enantiomer can have fatal or long-lasting side effects. Consequently, chiral pharmaceuticals that are produced synthetically must be prepared in enantiomerically pure form. The same is true of other chiral bio-active chemicals such as agrochemicals, pesticides and flavors. In recent years, researchers have also focused on the role of chirality in other areas such as catalysis [1], chiroptical applications [2,3], chiral magnetism [4], chiral semiconductors [5], cellular imaging [6], sensing [7], biomedical applications [8], chiral molecular assembly on surface [9], supramolecular chirality [10] and synthesis of chiral nanoparticles (NPs) [11]. One emerging field is chiral luminescent nanomaterials which have potential applications in chiral sensing [7]. Although a number of these studies have required synthesis of nanomaterials, it is still challenging to synthesize well-defined, monodispersed and inherently chiral NPs. In previous work, we have shown that L- and D-cysteine (Cys, HO2CCH(NH2)CH2SH) modified surfaces of Au-NPs are capable of enantiospecific adsorption of racemic propylene oxide (PO) [11,12] and the chiral pharmaceutical propranolol hydrochloride [13]. In these studies, Au-NPs were either spherical or tetrahexahedral (24-sided) in shape. A systematic study has also been performed to understand the effects of temperature and of Au-NP size on enantiospecific adsorption from racemic

Nanomaterials 2020, 10, 1716; doi:10.3390/nano10091716 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 1716 2 of 8 Nanomaterials 2020, 10, x 2 of 9

PO [[14].14]. These These studies studies made made use use of of optical polarimetry as a means of detecting detecting enantioselective enantioselective adsorption and used a model developed by us to to analyze analyze optical optical rotatio rotationn measurements measurements and quantify enantiospecificenantiospecific adsorption equilibrium constants.constants. In this work, we havehave usedused sphericalspherical Au nanoparticles,nanoparticles, which were synthesized based on our previous published work [[11].11]. We We chose to use spherical Au nanoparticlesnanoparticles based on our previousprevious work [[14]14],, which showed that the size of Au-NPsAu-NPs has an influenceinfluence on their adsorption of chiral compounds and and subsequent subsequent optical optical rotation. rotation. Smaller Smaller sized sized chiral chiral Au-NPs Au yield-NPs higher yield optical higher rotation optical duringrotation exposure during exposure to racemic to mixtures racemic ofmixtures chiral probe of chiral molecules. probe molecules. This indicates This that indicates smaller that chiral smaller NPs yieldchiral moreNPs yield effective more enantiomer effective enantiomer separations separations than large than NPs, large presumably NPs, presumabl as a resulty as a of result their of higher their surfacehigher surface area. In area. comparison In comparison to our to past our work, past work, we have we developed have developed this work this inwork two in directions. two directions. First, weFirst, have we changed have changed the chiral the surface chiral modifier surface used modifier to render used the to surfaces render of the Au-NPs surfaces enantioselective. of Au-NPs Dienantioselective.fferent chiral ligands Different have chiral diff ligandserent optical have rotationsdifferent optical and di ffrotationserent capacities and different for adsorption capacities and for interactionadsorption withand interaction chiral probe with molecules. chiral probe We have molecules. substituted We the have D- /substitutedL-Cys (Figure the1a) D used-/L-Cys as a(Figure chiral surface1a) used modifier as a chiral in our surface past work modifier [11–13 ] in with our D- past/L-penicillamine work [11–13] (Pen, with HO D2-CCH(NH/L-penicillamine2)C(CH3 ) (Pen,2SH) (FigureHO2CCH(NH1b). In our2)C(CH notation,3)2SH) D-(Figure/L-denotes 1b). In either our notation, pure enantiomer, D-/L- denotes while either DL-denotes pure enantiomer use of the racemic, while mixture.DL- denotes Penicillamine use of the was racemic chosen mixture. as chiral Penici modifierllamine because was its chosen chemical as chiral functionality modifier and because structure its arechemical similar functionality to those of and Cys. structure Both are are amino similar acids to those with alkylof Cys. thiol Both side are groups. amino acids Cys iswith one alkyl of the thiol 20 naturallyside groups. occurring Cys is chiralone of amino the 20 acidsnaturally that isoccurring polar in naturechiral amino and is theacids only that naturally is polar occurringin nature aminoand is acidthe only with naturally a thiol group occurring (-SH) [ amino15]. Adsorption acid with of a thiolCys on group Au surfaces (-SH) [15]. is dependent Adsorption on of the Cys pH on of theAu solutionsurfaces is [16 dependent]. At an acidic on the pH, pH Cys of the bonds solution to the [16]. Au At surface an acidic mainly pH, viaCys deprotonation bonds to the Au of thesurface -SH bond.mainly Bonding via deprotonation via the carboxylic of the -SH acid bond. group Bonding or amine via group the ca isrboxylic minimal, acid although group theor amine –NH2 group is + + convertedminimal, although to –NH3 the. On –NH the2 othergroup hand, is converted in alkaline to – pH,NH3 Cys. On still the bonds other with hand, the in Au alkaline surface pH, mainly Cys viastill -SH bonds bond with deprotonation the Au surface but alsomainly via via the – –NHSH 2bondgroup. deprotonation In alkaline pH, but the also carboxyl via the group–NH2 isgroup. ionized, In andalkaline some pH, studies the carboxy suggestl group that it is ionized aligned, paralleland some to thestudies Au surface.suggest Somethat it DFTis aligned studies parallel suggest to that the CysAu surface. can bond Some with DFT the Austudies surface suggest via both that thiolateCys can and bond amino-thiolate with the Au modes.surface Thevia both bond thiolate strength and of N–Auamino- andthiolate S–Au modes. were calculated The bond tostrength be 6 and of 47N– kcalAu /andmol, S respectively–Au were calculated [17]. Another to be di6 ffanderence 47 kcal/mol, between Penrespectively and Cys is[17]. their Another specific difference optical rotation. between The Pen specific and optical Cys is rotation their specific of Pen in optical water rotation. is ~9 higher The × thanspecific that optical of Cys. rotation Some studies of Pen in have water shown is ~9 that× higher Pen isthan a strong that of chelating Cys. Some agent studies and bonds have shown with many that metals,Pen is a particularlystrong chelating with agent Cu [18 and–20 ].bonds with many metals, particularly with Cu [18–20].

a) b)

Figure 1. The molecular structures of (a) L-Cys and (b) L-Pen differ only in the addition of two methyl Figure 1. The molecular structures of (a) L-Cys and (b) L-Pen differ only in the addition of two methyl groups to the terminal carbon. groups to the terminal carbon. The second direction in which this work expands on our prior accomplishments is by documenting The second direction in which this work expands on our prior accomplishments is by the effect of adsorption time on enantioselective adsorption of racemic RS-PO on chirally modified documenting the effect of adsorption time on enantioselective adsorption of racemic RS-PO on Au-NPs. In the past, enantioselective adsorption was studied within one hour of exposure of the chiral chirally modified Au-NPs. In the past, enantioselective adsorption was studied within one hour of NPs to the chiral probe PO [11–14]. Here, we find that the equilibration of PO on Pen/Au-NP requires exposure of the chiral NPs to the chiral probe PO [11–14]. Here, we find that the equilibration of PO ~18 h of exposure. Our work shows that when using Pen as a chiral ligand, only weak enantiomeric on Pen/Au-NP requires ~18 h of exposure. Our work shows that when using Pen as a chiral ligand, separation of PO is observed within one hour of PO exposure; however, there is an almost ten-fold only weak enantiomeric separation of PO is observed within one hour of PO exposure; however, increase in enantiomeric separation after 18 h of PO exposure. there is an almost ten-fold increase in enantiomeric separation after 18 h of PO exposure.

2. Materials and Methods

2.1. Synthesis of Au Nanoparticles Spherical Au-NPs were prepared using the synthesis described in our previous work [11,12]. Monodispersed Au-NPs were obtained by reduction of Au(III) chloride hydrate (HAuCl4•H2O) using

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2. Materials and Methods

2.1. Synthesis of Au Nanoparticles Nanomaterials 2020, 10, x 3 of 9 Spherical Au-NPs were prepared using the synthesis described in our previous work [11,12]. Monodispersedsodium borohydride Au-NPs were (NaBH obtained4) in by the reduction presence of Au(III) of chloride sodium hydrate citrate (HAuCl dihydrateH O) 4· 2 (HOC(COONa)(CH2COONa)22H2O) at room temperature. A typical synthesis involves preparation using sodium borohydride (NaBH4) in the presence of sodium citrate dihydrate (HOC(COONa)(CHof 200 mL of 2.5 × 10COONa)−4 M Au(III)2H chlorideO) at room hydrate temperature. in deionized A typical water. synthesis The solution involves was preparation stirred at of a 2 2· 2 200constant mL of rate 2.5 for10 54–M10 Au(III)min to chlorideobtain a hydrate light-yellow in deionized homogenous water. The solution. solution Next, was stirred0.015 g at of a constantsodium × − ratecitrate for 5–10dihydrate min to was obtain added a light-yellow to the Au(III) homogenous chloride solution. hydrate Next, solutio 0.015n. g Theof sodium color citrate of the dihydrate solution waschanged added from to the light Au(III) yellow chloride to colorless. hydrate Once solution. the change The color in color of the was solution observed, changed the stirring from light speed yellow was toaccelerated. colorless. Once Next, the 6 m changeL of ice in-cold color 0.1 was M observed, sodium theborohydride stirring speed was wasslowly accelerated. added to Next, the reaction 6 mL of ice-coldsolution 0.1 at Ma constant sodium borohydriderate over a period was slowly of one added minute. to the The reaction accelerated solution stirring at a constant was continued rate over for a periodanother of minute one minute. and then The the accelerated speed was stirring turned was to continuedmoderate forstirring. another The minute color of and the then reaction the speed solutio wasn turnedthen turned to moderate to wine stirring. red. Finally, The color the ofreaction the reaction solution solution was thenallowed turned to stir to wine for anoth red. Finally,er 5 h. The the reactionAu-NP solutionsolutions was were allowed stored to in stir glass for vials another and 5 covered h. The Au-NP with aluminum solutions were foil to stored prevent in glass degradation vials and by covered light. with aluminum foil to prevent degradation by light. 2.2. Coating of Au Nanoparticles with D- or L-penicillamine 2.2. Coating of Au Nanoparticles with D- or L-penicillamine Following synthesis, the Au-NP solution was diluted (3 mL of solution into 15 mL of deionized water),Following and 0.067 synthesis, g of D- or the L- Au-NPPen was solution added to was the diluted diluted (3 Au mL-NP of solutionsolution. into The 15final mL concentration of deionized water),of D- or and L-Pen 0.067 was g of0.025 D- orM. L-Pen The solution was added was to allowed the diluted to sonicate Au-NP for solution. 10 min The prior ﬁnal to concentrationuse. Once the ofAu D--NPs or L-Penwere coated was 0.025 with M. D- The or L solution-Pen and was then allowed exposed to to sonicate PO, the for solutions 10 min were prior allowed to use. Once to sit thefor Au-NPs18 h before were optical coated rotation with D- measurements or L-Pen and thenwere exposed taken. A to wait PO, time the solutions of 18 h was were chosen allowed after to sitinitial for 18experiments h before optical observed rotation that adsorption measurements of PO were on D taken.- or L A-Pen/Au wait time-NPs of reaches 18 h was a steady chosen state after after initial 18 experimentsh. observed that adsorption of PO on D- or L-Pen/Au-NPs reaches a steady state after 18 h.

2.3. Characterization of Au-NPsAu-NPs Transmission electronelectron microscopymicroscopy (TEM)(TEM) imagingimaging of the Au-NPsAu-NPs prepared using the procedure described above indicates that they are roughly spherical and about 3−55 nm in diameter (Figure 22).). − TheThe Au-NPs Au-NPs were also also characterized characterized using UV-VisUV-Vis spectroscopy. spectroscopy. In order to obtain the UVUV-Vis-Vis spectra, dilute solutions of Au-NPsAu-NPs were scanned over the range 400−990000 nm using a Cary UV UV-Vis-Vis − spectrophotometer (Agilent,(Agilent, SantaSanta Clara, Clara, CA, CA, USA). USA) A. A distinct distinct peak peak of of 513 513 nm nm was was observed, observed which, which is characteristicis characteristic of of 3 35−5 nm nm spherical spherical Au-NPs Au-NPs [4 [].4]. The The full full width width at at halfhalf maximummaximum (FWHM)(FWHM) ofof the peak − waswas alsoalso narrow,narrow, suggestingsuggesting thatthat thethe Au-NPsAu-NPs werewere fairlyfairly monodispersed.monodispersed.

FigureFigure 2.2. TEM image of Au-NPs.Au-NPs. Nanoparticles are roughly spherical.

2.4. Optical Rotation Measurements 2.4. Optical Rotation Measurements Optical rotation measurements were carried out usingusing aa RudolphRudolph ResearchResearch AnalyticalAnalytical AutopolAutopol VIVI (Hackettstown,(Hackettstown, NJ, USA).USA). This polarimeter can measure optical rotation of samplessamples at controlledcontrolled temperatures and at six different wavelengths. In a typical optical rotation measurement, 3 mL of sample solution was put into a Temptrol polarimeter sample cell (Hackettstown, NJ, USA). All measurements in this work were carried out using 436 nm wavelength light and at a temperature of 23 °C.

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Nanomaterialstemperatures 2020 and, 10 at, x six different wavelengths. In a typical optical rotation measurement, 3 mL of sample4 of 9 solution was put into a Temptrol polarimeter sample cell (Hackettstown, NJ, USA). All measurements The optical rotation measurement protocol focused on preparation of solutions with well- in this work were carried out using 436 nm wavelength light and at a temperature of 23 ◦C. controlled fractional concentrations of R-, S- or RS-PO in solutions of Pen/Au-NPs. In a typical set of The optical rotation measurement protocol focused on preparation of solutions with well-controlled optical rotation measurements, 14 scintillation vials were used. Seven vials had 6 mL of L-Pen/Au- fractional concentrations of R-, S- or RS-PO in solutions of Pen/Au-NPs. In a typical set of optical NPs and the other seven vials had 6 mL of D-Pen/Au-NPs. The solution was sonicated for 10 min. rotation measurements, 14 scintillation vials were used. Seven vials had 6 mL of L-Pen/Au-NPs and the After sonication, 0, 10, 20, 30, 40, 60 and 80 µL of R- PO was added to seven vials of L-Pen/Au-NPs. other seven vials had 6 mL of D-Pen/Au-NPs. The solution was sonicated for 10 min. After sonication, The concentrations of PO in the vials were 0, 0.023, 0.047, 0.07, 0.09, 0.14 and 0.19 M. This procedure 0, 10, 20, 30, 40, 60 and 80 µL of R- PO was added to seven vials of L-Pen/Au-NPs. The concentrations was repeated with D-Pen/Au-NPs. The solutions were then allowed to sit for 18 h to equilibrate R- of PO in the vials were 0, 0.023, 0.047, 0.07, 0.09, 0.14 and 0.19 M. This procedure was repeated PO adsorption. A similar protocol was used with S-PO and RS-PO. After 18 h, optical rotation of all with D-Pen/Au-NPs. The solutions were then allowed to sit for 18 h to equilibrate R-PO adsorption. solutions was measured using the temperature-controlled polarimeter. A similar protocol was used with S-PO and RS-PO. After 18 h, optical rotation of all solutions was Au(III) chloride hydrate (>99.9%), sodium borohydride, L-Pen (>99%), D-Pen (>99%) and sodium measured using the temperature-controlled polarimeter. citrate dihydrate (99%) were purchased from Sigma Aldrich (St. Louis, MO, USA). R- and S-PO Au(III) chloride hydrate (>99.9%), sodium borohydride, L-Pen (>99%), D-Pen (>99%) and sodium (>98%), and RS-PO (>98) were purchased from Alfa Aesar (Havervill, MA, USA). All chemicals were citrate dihydrate (99%) were purchased from Sigma Aldrich (St. Louis, MO, USA). R- and S-PO (>98%), used without further purification. and RS-PO (>98) were purchased from Alfa Aesar (Havervill, MA, USA). All chemicals were used without further purification. 3. Results 3. ResultsAll the enantiomeric separations and adsorption experiments were done after 18 h of Pen/Au- NP exposurAll thee enantiomeric to the PO probe. separations Figure and3 shows adsorption our rationale experiments for using were 18 done h for after PO 18probe h of adsorption. Pen/Au-NP Figureexposure 3 toshows the PO optical probe. rotation Figure3 byshows solutions our rationale of enantiomerically for using 18 h for pure PO D probe- or adsorption.L-Pen and D Figure- or L3- Pen/Aushows optical-NPs in rotation solutions by containing solutions of 0.023 enantiomerically M of RS-PO. pureThe concentration D- or L-Pen and of Au D- orin L-Penthese solutions/Au-NPs inis 0.24solutions M and containing the concentration 0.023 M of Pen RS-PO. is 0.025 The M. concentration Optical rotation of Au measurements in these solutions were is taken 0.24 Mat andregular the intervalsconcentration over ofa period Pen is 0.025of ~20 M. h, Opticalas shown rotation in Figure measurements 3. As seen from were the taken experiments, at regular intervalsduring the over first a period8 h the ofoptical ~20 h,rotation as shown cha innges Figure significantly3. As seen and from at a the roughly experiments, constant during rate. However, the first 8 after h the ~18 optical h of exposurerotation changes to the RS significantly-PO the optical and rotations at a roughly reach constant a steady rate. state. However, One thing after that ~18 is h evident of exposure from tothese the experimentsRS-PO the optical is that rotations the optical reach rotation a steady of state.D- or OneL-Pen/Au thing- thatNPs is is evident larger than from that these of experiments enantiomeric is purethat theD- opticalor L-Pen. rotation of D- or L-Pen/Au-NPs is larger than that of enantiomeric pure D- or L-Pen.

0.30 L-Pen/Au-NP 0.25

0.20

0.15

) L-Pen o 0.10

0.05

0.00

-0.05

OpticalRotation ( -0.10 D -Pen -0.15

-0.20

-0.25 D -Pen/Au-NP -0.30 0 2 4 6 8 10 12 14 16 18 20 Time (hr)

Figure 3. Optical rotation versus time by L- and D-Pen-coated Au-NPs and enantiomerically pure L- and D- Pen after exposure to a solution of 0.023 M RS-PO. The curves for D- and L-Pen are fits of a Figure 3. Optical rotation versus time by L- and D-Pen-coated Au-NPs and enantiomerically pure L- first-order exponential growth curve. The concentration of Au was 0.24 M and the concentration of Pen and D- Pen after exposure to a solution of 0.023 M RS-PO. The curves for D- and L-Pen are fits of a was 0.025 M. first-order exponential growth curve. The concentration of Au was 0.24 M and the concentration of PenOur was detection 0.025 M. of enantiospecific adsorption of chiral probes such as PO on chiral Au-NPs relies on measurements of the change in optical rotation observed during the addition of racemic RS-PO to Our detection of enantiospecific adsorption of chiral probes such as PO on chiral Au-NPs relies on measurements of the change in optical rotation observed during the addition of racemic RS-PO to solutions containing the chiral Au-NPs. In principle, the addition of a racemic probe to the solution should induce no change in optical rotation, unless that probe interacts enantiospecifically with the

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Nanomaterials 2020, 10, x 5 of 9 solutions containing the chiral Au-NPs. In principle, the addition of a racemic probe to the solution shouldchiral Au induce-NPs nosuch change that one in optical enantiomer rotation, adsorbs unless preferentially that probe interacts on the Au enantiospecifically-NP surface. Detection with the of chiralthis separation Au-NPs suchby optical that one polarimetry enantiomer also adsorbs requires preferentially that the specific on the optical Au-NP rotation surface. constant Detection of the of thisadsorbed separation probe byenantiomer optical polarimetry differs from also its requiresvalue in solution that the specific[12,13]. optical rotation constant of the adsorbedPrior probeto studying enantiomer the optical differs rotation from its induced value in by solution adding [ 12racemic,13]. RS-PO to solutions containing chiralPrior Au-NPs to studying, we have the studied optical the rotation rotation induced induced by by adding adding racemic enantiomerically RS-PO to solutions pure R- containingor S-PO to chiralsolutions Au-NPs, containing we have D- or studied L-Pen-coate the rotationd Au-NPs. induced Figure by 4 shows adding plots enantiomerically of optical rotation pure as R- a function or S-PO toof the solutions concentration containing of enantiomerically D- or L-Pen-coated pure Au-NPs. R- and S- FigurePO. The4 shows data for plots S-PO of are optical plotted rotation with solid as a functionsymbols ofand the reveal concentration positive optical of enantiomerically rotations at all pure concentrations R- and S-PO. and The in data the for presence S-PO are of plottedboth D- with and solidL-Pen/Au symbols-NPs. and It reveal is clear positive that the optical PO enantiomer rotations at all interacts concentrations enantiospecifically and in the presence with the of Au both-NPs D- andmodified L-Pen by/Au-NPs. the different It is clear enantiomers that the POof Pen. enantiomer Figure 4 interactsprovides enantiospecifically clear evidence that with some the fraction Au-NPs of modifiedthe enantiomerically by the different pure enantiomersPO in each solution of Pen. Figuremust interact4 provides with clear the evidencePen/Au-NPs. that As some shown, fraction the ofoptical the enantiomericallyrotation magnitude pure for PO pure in R each- and solution S-PO is must0.85 ± interact 0.01°/M with in close the agreement Pen/Au-NPs. with As our shown, prior the optical rotation magnitude for pure R- and S-PO is 0.85 0.01 /M in close agreement with our observations [11]. The optical rotation magnitude for S-PO with± L◦-Pen/Au-NPs is highest at ~2.9 ± prior0.07 °/M. observations This is roughly [11]. The 3.5 optical the specific rotation optical magnitude rotation for for S-PO enantiomerically with L-Pen/Au-NPs pure isS-PO. highest Note at that, ~2.9 0.07 /M. This is roughly 3.5 the specific optical rotation for enantiomerically pure S-PO. Note that, ±as required◦ by diastereomerism,× the magnitude of the optical rotation of R-PO in the presence of D- asPen/Au required-NPs by is diastereomerism,also roughly 3.5 the rotation magnitude by pure of the R optical-PO. In rotationcontrast, of S- R-POPO with in theD-Pen/Au presence-NPs of D-Pen/Au-NPs is also roughly 3.5 the rotation by pure R-PO. In contrast, S-PO with D-Pen/Au-NPs results in a suppression of optical× rotation to 0.3 ± 0.03 °/M. This implies that S-PO interaction with results in a suppression of optical rotation to 0.3 0.03 /M. This implies that S-PO interaction with the the D-Pen/Au-NPs reduces and even inverts the± optical◦ rotation of S-PO. The same inversion is D-Penobserved/Au-NPs for the reduces interactio andn evenof R-PO inverts with the L-Pen/Au optical- rotationNPs. of S-PO. The same inversion is observed for the interaction of R-PO with L-Pen/Au-NPs.

0.7 S -PO L-Pen/Au 0.6 2.89±0.06 o/M 0.5 0.85±0.01 o/M 0.4 0.31±0.03 o/M 0.3 ) o 0.2 S -PO 0.1 D -Pen/Au 0.0 L-Pen/Au -0.1 -0.2 R-PO

OpticalRotation ( -0.3 -0.29±0.03 o/M -0.4 o -0.84±0.01 /M -0.5 -2.91±0.07 o/M -0.6 R-PO D -Pen/Au -0.7 0.00 0.05 0.10 0.15 0.20 0.25 [PO] (M)

Figure 4. Optical rotation versus concentration of enantiomerically pure R- or S-PO in water and in solutionsFigure 4. containingOptical rotation L- or D-Penversus/Au-NPs. concentration In aqueous of enantiomerically solution, ( and pure), pure R- or R- S and-PO S-POin water rotate and light in • withsolutions a specific containing rotation L- of or 0.85D-Pen/Au0.01-NPs./M. InIn theaqueous presence solution, of L- or (● D-Pen and# /○Au-NPs,), pure R the- and optical S-PO rotation rotate ± ± ◦ forlight S-PO with changes a specific from rotation 2.9 to 0.3of ◦±0.85/M. The ± 0.01 concentration °/M. In the of presence Au was of 0.24 L- M or and D-Pen/Au the concentration-NPs, the optical of Pen wasrotation 0.025 for M. S-PO changes from 2.9 to 0.3 °/M. The concentration of Au was 0.24 M and the concentration of Pen was 0.025 M. It is important to remember that the solutions containing Pen/Au-NPs also contain some free Pen enantiomersIt is important that are to not remember adsorbed t onhat the the Au-NP solutions surfaces. containing PO added Pen/Au to the-NPs solutions also contain can, in some principle, free interactPen enantiomers with both that the are Pen not/Au-NPs adsorbed and on with the the Au free-NP Pen.surfaces. To probePO added the magnitude to the solutions of this can, effect, in weprinciple, conducted interact the with same both experiment the Pen/Au as illustrated-NPs and with in Figure the free4 but Pen. without To probe the the presence magnitude of Au-NPs. of this Theeffect results, we conducted are shown the in same Figure experiment5. The magnitude as illustrated of the in optical Figure rotations4 but without of S-PO the withpresence L-Pen of andAu- R-PONPs. The with results D-Pen are are shown ~2.1 in0.05 Figure/M, 5. lower The magnitude than the optical of the rotation optical observedrotations of for S PO-PO inwith solutions L-Pen ± ◦ containingand R-PO with Pen/ DAu-NPs.-Pen are It ~2. is1 important ± 0.05 °/M, to lower point than out that the thisoptical diff rotationers from observed our prior for work PO using in solutions Cys as containing Pen/Au-NPs. It is important to point out that this differs from our prior work using Cys as the chiral surface modifier. PO shows no enantiospecific interaction with Cys in the solution phase [9]. In contrast, S-PO with D-Pen and R-PO with L-Pen show optical rotation magnitude of 0.5 ± 0.02

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theNanomaterials chiral surface 2020, modifier.10, x PO shows no enantiospecific interaction with Cys in the solution phase6 of 9 [ 9]. In contrast, S-PO with D-Pen and R-PO with L-Pen show optical rotation magnitude of 0.5 0.02 /M ± ◦ which°/M which is greater is greater than than that that observed observed for for S-PO S-PO interacting interacting with with D-Pen D-Pen/Au/Au-NPs.-NPs. The The second second set set of of experimentsexperiments in in Figure Figure5 uses5 uses R-PO R-PO as as the the probe probe andand confirmsconfirms the res resultsults obtained obtained with with S- S-PO.PO. In In both both cases,cases, the the interactions interactions with with Pen Pen/Au/Au-NPs-NPs induceinduce greater changes changes in in magnitude magnitude of of the the optical optical rotations rotations thanthan observed observed with with just just the the modifier modifier in in solution; solution; i.e. i.e.,, the the shift shift in in optical optical rotation rotation arises arises from from the the interactioninteraction of of PO PO with with both both Pen Pen/Au/Au-NPs-NPs andand PenPen inin solution.

0.7 S-PO 0.6 0.5 2.05±0.05 o/M L-Pen 0.4 0.85±0.01 o/M o 0.3 0.52±0.01 /M ) o 0.2 S-PO 0.1 D -Pen 0.0

-0.1 L-Pen -0.2 R-PO

OpticalRotation ( -0.3 -0.52±0.02 o/M -0.4 -0.84±0.01 o/M o D -Pen -0.5 -2.09±0.05 /M -0.6 R-PO -0.7 0.00 0.05 0.10 0.15 0.20 0.25 [PO] (M)

Figure 5. Optical rotation by enantiomerically pure R- and S-PO in water and in aqueous solutions of enantiomerically pure L- or D-Pen. In aqueous solution ( and ), pure R- and S-PO rotate light with a Figure 5. Optical rotation by enantiomerically pure R- and• S-PO in water and in aqueous solutions of specific rotation of 0.85 0.01 /M. In the presence of enantiomerically# pure L- or D-Pen, the optical enantiomerically ±pure L-± or D-Pen.◦ In aqueous solution (● and ○), pure R- and S-PO rotate light with rotationa specific of S-POrotation changes of ± 0.8 to5 ± 2.1 0.01 and °/M. 0.5 In◦ /theM, presence respectively. of enantiomerically The optical rotations pure L- or of D R-PO-Pen, are the relatedoptical to thoserotation of S-PO of S by-PO diastereomerism. changes to 2.1 and The 0.5 concentration°/M, respectively. of D- The and optical L-Pen rotations was 0.025 of R M.-PO are related to those of S-PO by diastereomerism. The concentration of D- and L-Pen was 0.025 M. The clearest demonstration of the enantiospecific interaction of the PO with chirally modified Pen/Au-NPsThe clearest comes demonstration from measurement of the ofenantiospecific optical rotation interaction of light duringof the PO addition with chirally of racemic modified RS-PO toPen/Au solutions-NPs containing comes from D- measurement or L-Pen/Au-NPs. of optical The rotation results of are light shown during in addition Figure6 .of In racemic the absence RS-PO of enantiospecificto solutions containing interactions, D- theor L addition-Pen/Au- ofNPs. a racemic The results compound are shown to a solutionin Figure would 6. In the not absence be expected of toenantiospecific cause any change interactions, in optical the rotation. addition As of a Figure racemic6 shows, compound this to is thea solution case for would addition not be of expected RS-PO to a solutionto cause any containing change in racemic optical DL-Penrotation./Au-NPs. As Figure However, 6 shows, this Figure is the6 showscase for clear addition changes of RS in-PO optical to a rotationsolution during containing addition racemic of RS-PO DL-Pen/Au to solutions-NPs. However, containing Figure either 6 shows D- or L-Penclear changes and during in optical RS-PO additionrotation to during solutions addition containing of RS-PO either to solutions D- or L-Pen containing/Au-NPs. either When D- or RS-PO L-Pen is and added during to a RS solution-PO containingaddition Au-NPs to solutions modified containing by DL-Pen either at D- a or concentration L-Pen/Au-NPs. of 0.025 When M, RS there-PO is no added net optical to a solution rotation, ascontaining both RS-PO Au and-NPs DL-Pen modified/Au-NPs by DL have-Pen at no a netconcentration chirality. However, of 0.025 M addition, there is no of RS-POnet optical to the rotation solution, as both RS-PO and DL-Pen/Au-NPs have no net chirality. However, addition of RS-PO to the solution containing pure D-Pen/Au-NPs induced a rotation of 1.50 ◦/M(), while addition of RS-PO to the containing pure D-Pen/Au-NPs induced a rotation of −1.50 °/M (□), while addition of RS-PO to the solution containing pure L-Pen/Au-NPs induced a rotation of +1.49 ◦/M(). The magnitudes of the ■ opticalsolution rotation containing observed pure during L-Pen/Au addition-NPs induced of RS-PO a rotation to solutions of +1.49 containing °/M ( ). D-The or magnitudes L-Pen/Au-NPs of the are optical rotation observed during addition of RS-PO to solutions containing D- or L-Pen/Au-NPs are almost twice the optical rotation observed during addition of RS-PO to solutions containing 0.025 M D- almost twice the optical rotation observed during addition of RS-PO to solutions containing 0.025 M or L-Pen (∆ andN, respectively) with no Au-NPs. D- or L-Pen (Δ and▲, respectively) with no Au-NPs. The structures of L-/D-Pen and L-/D-Cys are very similar, differing only in that Pen has two methyl groups attached to its side chain. However, enantioselective adsorption of PO on Cys- and Pen-coated Au-NPs differs in three major ways. The first is that Cys-coated Au-NPs show steady-state enantioselective adsorption of PO after one hour of exposure. However, in the case of Pen-coated Au-NPs we require 18 h of Au-NP exposure to the PO probe, in order to observe steady-state adsorption. This suggests that Cys reaches a steady-state coverage and conformation on Au much more rapidly than Pen.

Nanomaterials 2020, 10, 1716 7 of 8

The second diﬀerence between the two chiral modiﬁcation schemes is the optical rotation intensity. Cys-coatedNanomaterials Au-NPs 2020, 10, x have lower optical rotations for enantioselective adsorption of racemic PO7 of than 9 Pen-coated Au-NPs.

0.45

0.40 o 1.49±0.05 /M 0.35 L-Pen/Au 0.30 0.63±0.02 o/M 0.25 0.20 L-Pen 0.15 ) o 0.10 0.05 0.00 DL-Pen/Au -0.05 -0.10 -0.15 D-Pen

OpticalRotation ( -0.20 -0.25 -0.63±0.02 o/M -0.30 o -0.35 -1.50±0.04 /M D-Pen/Au -0.40 -0.45 0.00 0.05 0.10 0.15 0.20 0.25 [rac-PO] (M)

Figure 6. Optical rotation versus RS-PO concentration during addition to solutions containing Au-NPs coatedFigure with 6. Optical L-Pen rotation (), D-Pen versus ( )RS and-PO DL-Penconcentration (?). during Addition addition of RS-PO to solutions to solutions containing containing Au- enantiomericallyNPs coated with pure L-Pen D-Pen (■), D (-∆Pen) and (□) L-Penand DL (N-Pen) indicates (⋆). Addition some of interaction RS-PO to betweensolutions POcontaining and Pen. Theenantiomerically concentration ofpure Au D was-Pen 0.24 (Δ) Mand and L-Pen the ( concentration▲) indicates some of D-, interaction L- and DL-Pen between was PO 0.025 and Pen. M. The concentration of Au was 0.24 M and the concentration of D-, L- and DL-Pen was 0.025 M. The third difference between Cys and Pen as chiral modifiers of Au/NP surfaces is clearly shown in TableThe1. Table structures1 compares of L-/D the-Pen optical and rotationsL-/D-Cys ofare pure very R-, similar, S- and racemicdiffering PO only on in Cys- that and Pen Pen-coated has two Au-NPs.methyl Thegroups optical attached rotations to its byside R- chain. and S-POHowever, on L- enantioselective or D-Cys-coated adsorptio Au-NPsn of are PO almost on Cys identical.- and However,Pen-coated there Au is-NPs a significant differs in di threefference major in ways. the optical The first rotation is that of Cys R- and-coated S-PO Au on-NPs L- orshow D-Pen-coated steady- Au-NPs.state enantioselective S-PO has higher adsorption optical rotationof PO after with one L-Pen-coated hour of exposure. Au-NPs However, and lower in the on case D-Pen-coated of Pen- Au-NPs.coated Au On-NPs the otherwe require hand, 18 R-PO h of Au shows-NP theexposu oppositere to the eff POect. probe, R-PO in has order higher to observe optical steady rotation-state with D-Pen-coatedadsorption. This Au-NPs suggests and lowerthat Cys optical reaches rotation a steady L-Pen-coated-state coverage Au-NPs. and conformation on Au much more rapidly than Pen. TableThe 1. secondSpecific difference optical rotation between during the addition two chiral of R-, S-modification and RS-PO to schemes solutions is of the Au-NPs optical that rota havetion intensity.been chirally Cys-coated modified Au with-NPs D- have or L-Pen,lower oroptical D- or rotations L-Cys. for enantioselective adsorption of racemic PO than Pen-coated Au-NPs. TheOptical third Rotation difference (◦/ M)between D-Pen Cys/ Au-NPand Pen as L-Pen chiral/Au-NP modifiers D-Cysof Au/NP/Au-NP surfaces L-Cys is clearly/Au-NP shown in Table 1. TableS-PO 1 compares the 0.31optical0.03 rotations 2.89 of pure0.06 R-, S- and 1.69 racemic0.10 PO on 1.80 Cys-0.06 and Pen- ± ± ± ± coated Au-NPs.R-PO The optical rotations2.91 by0.07 R- and S0.31-PO on0.03 L- or D1.66-Cys-coated0.05 Au-1.75NPs are0.05 almost identical. However, there is a significant− ± difference− in the± optical rotation− ± of R- and− S-PO± on L- or D- RS-PO 1.50 0.04 1.49 0.05 0.15 0.01 0.15 0.01 Pen-coated Au-NPs. S-PO has higher− ±optical rotation± with L-Pen-coated± Au-NPs −and lower± on D- Pen-coated Au-NPs. On the other hand, R-PO shows the opposite effect. R-PO has higher optical 4.rotation Conclusions with D-Pen-coated Au-NPs and lower optical rotation L-Pen-coated Au-NPs. Our work suggests that enantioselective adsorption of PO on Cys-modified Au-NPs is different Table 1. Specific optical rotation during addition of R-, S- and RS-PO to solutions of Au-NPs that have than on Pen-modified Au-NPs. Enantiomerically pure R- or S-PO shows different optical rotations on been chirally modified with D- or L-Pen, or D- or L-Cys. D- or L-Pen-coated Au-NPs. On the other hand, enantioselective adsorption of pure R- or S-PO on D- or L-Cys-coatedOptical Au-NPs is the same. In addition, the specific optical rotation observed for RS-PO on D- or L-Pen-coatedRotation Au-NPsD-Pen/Au is 10-NP higherL than-Pen/Au during-NP adsorptionD-Cys/ onAu D--NP or L-Cys-coatedL-Cys/Au Au-NPs.-NP (°/M) × Author Contributions:S-PO Conceptualization,0.31 ± 0.03 N.S.2.89 and ± 0.06 A.J.G.; validation,1.69 ± N.S.0.10 and Z.B.;1.80 investigation, ± 0.06 Z.B.; writing—original draft preparation, N.S.; writing—review and editing, N.S. and A.J.G.; supervision, N.S.; funding acquisition,R-PO N.S.−2.91 and A.J.G. ± 0.07 All authors−0.31 have ± read 0.03 and agreed−1.66 to the ± published 0.05 version−1.75 of the± 0.05 manuscript. RS-PO −1.50 ± 0.04 1.49 ± 0.05 0.15 ± 0.01 −0.15 ± 0.01 Funding: This research was funded by the US NSF, grant number CHE1764252, a Carnegie Mellon University SURG/SURF grant for undergraduate research and by the Pennsylvania Infrastructure Technology Alliance (PITA). The4. APCConclusions was funded by Carnegie Mellon University.

Nanomaterials 2020, 10, 1716 8 of 8

Conﬂicts of Interest: The authors declare no conﬂict of interest.

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