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Supramolecular Chemistry

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Spectroscopic characterization and in silicomodelling of polyvinylpyrrolidone as an anion-responsive fluorescent polymer in aqueous media

Hiu C. Kam, Dineli T. S. Ranathunga, Ethan R. Payne, Ronald A. Smaldone,Steven O. Nielsen & Sheel C. Dodani

To cite this article: Hiu C. Kam, Dineli T. S. Ranathunga, Ethan R. Payne, Ronald A. Smaldone,Steven O. Nielsen & Sheel C. Dodani (2019): Spectroscopic characterization and in�silicomodelling of polyvinylpyrrolidone as an anion-responsive fluorescent polymer in aqueous media,Supramolecular Chemistry, DOI: 10.1080/10610278.2019.1630740

To link to this article: https://doi.org/10.1080/10610278.2019.1630740

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Spectroscopic characterization and in silico modelling of polyvinylpyrrolidoneas an anion-responsive fluorescent polymer in aqueous mediaHiu C. Kam , Dineli T. S. Ranathunga *, Ethan R. Payne , Ronald A. Smaldone , Steven O. Nielsenand Sheel C. Dodani

Department of Chemistry and Biochemistry, The University of Texas at Dallas, Richardson, TX, USA

ABSTRACTAqueous anion recognition has been a long-standing challenge in molecular recognition. The useof synthetic polymers is an emerging area of interest due to their conformational flexibility,creating microdomains that favor the desolvation and binding of anions in water. Here, we reportthat the fluorescence of off-the-shelf polyvinylpyrrolidone (PVP) can be used to detect thepresence of not only nitrate, as previously reported, but also nitrite, iodide, and thiocyanate inaqueous media. The extent of quenching and anion affinity is dependent on the solution pH andthe molecular weight of PVP, while showing close correlation with the Hofmeister series.Moreover, molecular dynamics simulations support our experimental findings, suggesting thatanions associating closest to the surface of PVP quench its fluorescence to the greatest extent.Combined with the versatility of PVP, this fundamental study provides a starting point to conferanion binding properties with a fluorescence output in new materials.

ARTICLE HISTORYReceived 22 April 2019Accepted 4 June 2019

KEYWORDSAqueous anion recognition;Hofmeister series;fluorescent polymer;polyvinylpyrrolidone;molecular dynamics

Introduction

The detection of anions in water is a long-standingchallenge in the field of molecular recognition (1–3).Specifically, displacing the water hydration shell ofanions by disrupting the hydrogen-bonding networkrequires the host to overcome a large thermodynamicbarrier (1–3). One underexplored approach to addressthis challenge is with the use of synthetic organic poly-mers (3–6). Polymers can have microdomains of hydro-phobicity and hydrophilicity which are capable ofmultivalent interactions with anions through Lewisacid/base, ion-pairing, and hydrogen, halogen, orπ-bonding (3, 5, 7, 8). When properly designed, thesemicrodomains can induce desolvation and promotefavorable binding of anions in water. Furthermore,

designer polymers can be customized with anion recep-tors to allow for selective detection of an anion bytargeting its size, shape, or charge (9). Upon anionbinding, a particular output can be afforded such ascolorimetric, fluorescence, temperature-response, orphase change in water (3–6, 10–26). While designerpolymers can be tuned for high specificity, this typicallycomes at the expense of time, cost, and scalability. Onthe other hand, commercially available polymers pro-vide economical and readily accessible alternatives.Oftentimes, these polymers have well-studied proper-ties that are useful for a wide range of applications,including anion recognition in water (27–31). For exam-ple, polyvinylpyrrolidone (PVP) is an off-the-shelf, water-soluble, and biocompatible polymer that is best known

CONTACT Sheel C. Dodani sheel.dodani@utdallas.edu Department of Chemistry and Biochemistry, The University of Texas at Dallas, Richardson, TX75080, USA*H.C.K. and D.T.S.R contributed equally to this work

Supplemental data for this article can be accessed here.

SUPRAMOLECULAR CHEMISTRYhttps://doi.org/10.1080/10610278.2019.1630740

© 2019 Informa UK Limited, trading as Taylor & Francis Group

for its use in the topical delivery of iodine in the form oftriiodide as an antiseptic (32, 33).

Early halide NMR line broadening experiments haveestablished that polyolefin polymers, such as PVP,poly(2-hydroxyethyl methacrylate) (iso-PHEMA), polyacry-lamide (PAAM), polyethylene glycol (PEO), polyvinyl (vinylalcohol-co-vinyl acetate) (PVAL), and polyacrylic acid (PAA)can bind iodide in water (34). Interestingly, of these poly-mers, PVP has the strongest affinity for iodide eventhough it lacks an NH moiety (29). Subsequent directand competitive halide NMR studies indicate that theaffinity of PVP can be ranked as follows: SO4

2- < F− < Cl−

< NO3− < Br− < I− < SCN− (35). This trend tracks with the

Hofmeister series derived from ranking the bulk hydrationbehavior of ions with proteins (36, 37). Anion binding toPVP can also be observed through macroscopic changesin viscosity and lower critical solution temperature orsalting out (38–40). More recent studies show similaranion binding with poly(N-isopropylacrylamide)(PNIPAM) and poly(N,N-diethylacrylamide) (PDEA), withthe latter lacking a free NH moiety (27–30).

One unique property that distinguishes PVP is itsfluorescence in the solid state and in water despitelacking conjugation (41, 42). Evidence suggests thatthis fluorescence could arise from an aggregationinduced emission (AIE) phenomena (42). Interestingly,the fluorescence of PVP is quenched upon the additionof metal ions and nitrate in water, but the mechanismof quenching is unknown (42, 43). However, otheranions that are known to bind PVP based on halideNMR have not been reported to affect its fluorescenceoutput. Here, we show that commercially available PVPcan be a general turn-off fluorescent platform for thedetection of nitrite, nitrate, iodide, and thiocyanate inaqueous media. The extent of quenching and anionaffinity are dependent on both the pH of the solutionand molecular weight of PVP. Moreover, through mole-cular dynamics (MD) simulations, we link the observedanion selectivity to anion interactions at the polymersurface and concomitant changes to the polymeraggregation state.

Methods

General

Reagents and chemicals were purchased from Sigma-Aldrich (St. Louis, MO), TCI America (Portland, OR), orThermo Fisher Scientific (Waltham, MA) and wereused as received unless otherwise stated.Abbreviations for the different molecular weights ofpolyvinylpyrrolidone (PVP) used in this study arePVP40 (average MW = 40,000 g/mol), K15

(average MW = 10,000 g/mol), and K90 (average MW

= 360,000 g/mol).

General spectroscopic materials and methods

All spectroscopic measurements were carried out witha 10 × 10 mm quartz cuvette (3 mL) (catalog number:41FLUV10; FireflySci, Staten Island, NY) or 2 mm x 10 mmquartz cuvette (400 µL) (catalog number: 115-F-10-40;Hellma USA, Plainview, NJ) on a Horiba Fluorolog-3equipped with a 450-W xenon short arc lamp sourceand power supply, a R928P photomultiplier tube, andtwo Spex monochromator gratings (catalog number:5500000439; Horiba, Kyoto, Japan) at room temperature.All fluorescence excitation spectra were collected from200 nm to 360 nm (5-nm slit width) with emission at390 nm. Fluorescence emission spectra were collectedfrom 330 nm to 600 nm (5-nm slit width) with excitationprovided at 310 nm.

Anion screening of PVP40 fluorescence response inultrapure water

An 8 wt% solution of PVP40 in ultrapure water wasdiluted to 1 wt% with ultrapure water or ultrapurewater containing varying concentrations of sodiumcitrate, sodium sulfate, sodium phosphate, sodium acet-ate, potassium chloride, sodium chloride, sodiumnitrite, sodium nitrate, sodium chlorate, sodium bro-mide, sodium iodide, and potassium thiocyanate. Thefractional quenching (Ff/F0) was determined by dividingthe fluorescence intensity at 390 nm of PVP40 with400 mM of the anion tested (Ff) by the initial fluores-cence intensity of PVP40 without the anion (F0). Theaverage of three technical replicates with the standarddeviation is reported in Figures S1 and S2.

Spectroscopic characterization of PVP40 responseto anions as a function of pH

An 8 wt% solution of PVP40 in 50 mM sodium citratebuffer (pH 4), 50 mM sodium citrate buffer (pH 6), or50 mM potassium phosphate buffer (pH 8) was dilutedto 1 wt% with the corresponding buffer or buffer contain-ing varying concentrations of sodium chloride, sodiumnitrite, sodiumnitrate, sodium iodide, and potassium thio-cyanate. The fractional quenching (Ff/F0) was determinedby dividing the fluorescence intensity at 390 nm of PVP40with 400 mM of the anion tested (Ff) by the initial fluores-cence intensity of PVP40without the anion (F0). The appar-ent dissociation constant (Kd) for each anion wasdetermined by plotting the fractional binding [F = (Fobs –Fmin)/(Fmax – Fmin)] versus the anion concentration [A] in

2 H. C. KAM ET AL.

KaleidaGraph v4.5 (Synergy Software, Reading, PA) whereFobs is the observed fluorescence intensity at 390 nm andFmin and Fmax are the fluorescence intensities with 400mMand 0 mM anion, respectively. The apparent Kd for eachanion was calculated using the following equation: F = 1 –([A]/(Kd + [A])). The average of three technical replicateswith the standard deviation is reported in Figures S3 andS4 and summarized in Table S1.

Spectroscopic characterization of PVP response toanions as a function of molecular weight

An 8 wt% solution of K15 or K90 in 50 mM sodium citratebuffer (pH 6) was diluted to 1 wt% with the correspondingbuffer or buffer containing varying concentrations ofsodium nitrite, sodium nitrate, sodium iodide, and potas-sium thiocyanate. The fractional quenching (Ff/F0) wasdetermined by dividing the fluorescence intensity at390 nm of K15 or K90 with 400 mM of the anion tested(Ff) by the initial fluorescence intensity of K15 or K90 with-out the anion (F0). The apparent Kd for each anion wasdetermined as described above. The average of three tech-nical replicates with the standard deviation is reported inFigures S5 and S6 and summarized in Table S2.

Fully atomistic molecular dynamics simulations ofPVP

Eight different simulations of aqueous PVP in different ionicenvironments were carried out using the NAnoscaleMolecular Dynamics (NAMD) software package (v.2.12)(44). NAMD was developed by the Theoretical andComputational Biophysics Group in the Beckman Institutefor Advanced Science and Technology at the University ofIllinois at Urbana-Champaign (http://www.ks.uiuc.edu/Research/namd/). To create the initial model, nine linearpolymer chains of fifty monomers each were placed alongthe x-direction in a 3 × 3 bundle (Figure S7A). Periodicboundary conditions were employed in all three directions.The systemwas solvated and ionized at 1.1M ionic strengthfor all the monovalent anionic systems and at 3.3 M ionicstrength for the divalent sulfate system using the VisualMolecular Dynamics (VMD) software package (v.1.9.3)(http://www.ks.uiuc.edu/Research/vmd/) (45). The eight dif-ferent systems used sulfate, chloride, nitrite, nitrate, bro-mide, iodide, and thiocyanate with sodium cations, as wellas a control system without ions. To evolve the dynamics,weused anon-polarizable force field as opposed to ab initioMD or a polarizable force field. Although both alternativescan provide amore accurate ion solvation structure (46), abinitiomethods are limited to a few hundred atoms and thuscannot model realistic polymer conformations, while polar-izable force fields show minor improvements at best and

are not available for the wide range of ions studied here(47–50). The CHARMM General Force Field (CGenFF) para-meters were used for the PVP monomer unit and the addi-tional parameters required to link the monomers intoa polymer were fit using the online server at https://cgenff.paramchem.org. The CHARMM non-polarizableforce field parameters were used for sodium and chlorideions alongwith the TIP3Pwatermodel (51). Since CHARMMdoes not include parameters for sulfate, nitrite, nitrate,bromide, iodide, and thiocyanate ions, the necessary forcefield parameters and partial charges were adopted from(52–56) and are summarized in Table S3.

Following relaxation runs (Figure S7B) in which the initi-ally extended polymer chains contract, become entangled,and undergo a 50% decrease in their solvent accessiblesurface area (SASA), 200 ns NPT equilibrium simulationswere performed at 293 K and 1 atm using a 1 fs timestep.A 12 Å cutoff was used for the van der Waals and short-ranged electrostatics interactions, while the long-rangeelectrostatics were treated by the ParticleMesh Ewald tech-nique. Based on the MD trajectories, we generated theradial distribution function (RDF) for anions around thenitrogen (N), oxygen (O), andcarbon (C) atoms in the lactambond of PVP (Figure 4(a)). Distributions of different atoms inthe ions, as well as the center of mass of the ions, arounddifferent PVP atoms (N, O, C) were obtained. To provideadditional detail to the RDFs, the three-dimensional spatialdistribution of ionswithin 10Å fromany pyrrolidonemono-mer was calculated by transforming the ion coordinates toa relative coordinate system based on the monomer loca-tion and orientation. The resulting distribution of ion coor-dinates was analyzed with VMD’s VolMap tool to givedensity isosurfaces (Figure S8 and Table S4). To quantifythe polymer aggregation state, the solvent accessible sur-face area of PVP was calculated using VMD with a proberadius of 1.4 Å; periodic boundary conditionswere explicitlytaken into account with a custom TCL script.

Results and discussion

Consistent with that previously reported, an aqueoussolution of 1 wt% PVP40 (average MW = 40,000 g/mol)does not have an absorption peak in the UV-visibleregion, but does have a fluorescence excitation max-imum at 310 nm and an emission maximum at390 nm (Figure 1) (42). We also observe the Ramanscattering peak of water in both the excitation andemission spectra at 336 nm and 350 nm, respectively.We first screened the emission response of 1 wt%PVP40 to a panel of anions of varying size, shape,and charge in ultrapure water (Figures S1 and S2).Fluorescence quenching at 390 nm is not onlyobserved with 100 mM nitrate as reported with PVP

SUPRAMOLECULAR CHEMISTRY 3

K60 in water, but also with nitrite, iodide, and thio-cyanate (43). Little to no change in fluorescenceintensity is observed in the presence of chloridewith a sodium or potassium counterion, indicatingthat fluorescence quenching is unrelated to the cationor ionic strength (Figure S1E and S1F). However, anincrease in the ionic strength can alter the pH ofwater and owing to the fact that the emission ofPVP40 is pH dependent, we carried out further char-acterization in aqueous solutions buffered withsodium citrate and potassium phosphate, which donot affect the fluorescence of PVP in ultrapure water(Figure S1A and S1C) (42, 57). These pH controls havenot been previously carried out with PVP K60.

Compared to ultrapure water, the fluorescenceexcitation and emission maxima of 1 wt% PVP40 donot change in pH 6 buffer or upon the addition of400 mM chloride (Figure 1(b)). Titration with increas-ing concentrations of nitrate to a point of saturationat 400 mM shifts the emission maximum from 390 nmto 398 nm and quenches the emission by ca. 91%(Figure 1(c) and Table S1). Varying degrees of emis-sion shifts and fluorescence quenching are alsoobserved upon the addition of nitrite (λem= 400 nm, 99% turn-off), iodide (λem = 390 nm, 76%turn-off), and thiocyanate (λem = 390 nm, 39% turn-off) (Figure S3 and Table S1). With this titration data,the apparent dissociation constant (Kd) for each anionbinding to 1 wt% PVP40 was best fitted to a singlesite binding model as described above in theMethods section (Figure S4). This model sufficientlydescribes the diffusion-limited environment of PVPwith anions, and the irreversibility of fluorescencequenching given that the anion concentration is inexcess of up to ca. 2,000-fold. The trend of the Kdvalues does not correlate with the degree of quench-ing where SCN− < I− < NO3

− < NO2− and can be

ranked at pH 6 from smallest (high affinity) to largest(low affinity) as follows: NO2

− (Kd = 54 ± 4.9 mM) < I−

(Kd = 60 ± 4.1 mM) < SCN− (Kd = 75 ± 5.8 mM) < NO3−

(Kd = 172 ± 24 mM) (Table S1).We further evaluated the emission response of 1

wt% PVP40 as a function of pH upon the addition of400 mM anion. The degree of quenching is within 1%for nitrite and nitrate across pH 4, 6, and 8 (Figures 2(a)and S3, Table S1). The addition of thiocyanate quenchesthe emission by 50% at pH 4, 39% at pH 6, and 42% atpH 8. Interestingly, the addition of iodide significantlyquenches the emission by 99% at pH 4 when comparedto 76% at pH 6 and 56% at pH 8. This is likely due to theoxidation of iodide to triiodide in acidic conditions andsubsequent binding of triiodide to PVP (33). The

Figure 1. (a) Molecular structure of polyvinylpyrrolidone (PVP)with excitation (λem = 390 nm, red) and emission (λex= 310 nm, blue) fluorescence spectra of 1 wt% PVP40(average MW = 40,000 g/mol) in water. Fluorescenceresponse of 1 wt% PVP40 to (b) chloride and (c) nitrate.Spectra were acquired in 50 mM sodium citrate (pH 6) in thepresence of 0, 25, 50, 100, 200, and 400 mM (bold) chlorideor nitrate. Arrow direction corresponds to increasing anionconcentrations. (d) Anion selectivity of 1 wt% PVP40 in50 mM sodium citrate (pH 6). Emission response at 390 nm(λex = 310 nm) to 0 (F0, black bar), 25, 50, 100, 200, and400 mM (Ff, white bar) chloride, nitrite, nitrate, iodide, andthiocyanate. The average of three replicates is reported here.All spectra are shown in Figure S3.

4 H. C. KAM ET AL.

apparent Kd values for nitrate and iodide fall withinerror across pH 4, 6, and 8 while the apparent Kd valuesfor nitrite (Kd = 54 ± 4.9 mM) and thiocyanate (Kd

= 75 ± 5.8 mM) are higher at pH 6 and fall withinerror across pH 4 and 8 (Figure 2(b)). Under the condi-tions tested, the lactam bonds in PVP do not hydrolyze,therefore PVP lacks any free hydrogen atoms that cancontribute to the differences observed. Instead, thedecrease in intensity of the Raman peaks suggeststhat there is a reorganization of water and its hydrogenbonding network at the hydrophobic surface of PVPwhich is influenced by changes in pH (Figure S3).

To understand the effect of molecular weight onthe emission response of PVP to anions, we selectedPVP K15 (average MW = 10,000 g/mol) and PVP K90(average MW = 360,000 g/mol). Each PVP at 1 wt% hasapproximately the same number of N-vinyl pyrroli-done monomers (2.17x1019) (Table S5). At pH 6,neither PVP K15 nor PVP K90 have an absorptionpeak in the UV-visible region, but they havea fluorescence excitation maximum at 310 nm withan emission maximum at 390 nm and 380 nm, respec-tively (Figure S5). The relative emission intensity doesdepend on molecular weight and can be ranked asfollows: PVP K90 < PVP40 < PVP K15. Within eachmolecular weight of PVP, the degree of quenchingfollows the same trend (NO2

− < NO3− < I− < SCN−),

with the exception of PVP K90 with iodide (Figures 2(c) and S5, Table S2). Across the molecular weights,the degree of quenching falls within 5% except forPVP K90 with iodide; however, the apparent Kd valuesdo depend on molecular weight (Figures 2(d) and S6,Table S2). Based on this data, the affinity is greatestfor nitrite with K15, thiocyanate with PVP40, andnitrate and iodide with K90. Taken together, weobserve that the turn-off emission response andapparent Kd values are not only a function of theanion concentration and pH but also of the molecularweight of the polymer.

These anion specific effects can be better understoodthrough the Hofmeister series that is described as follows:SO4

2- < HPO42- < F- < CH3COO

- < C6H5O73- < Cl- < Br- <

NO3- ≈ NO2- < ClO3

- < I- < SCN- (36, 58-60). The anionslisted to the left of chloride are classified as kosmotropes.Their hydrophilic character enhances water-water interac-tions and perturbs the hydration shell around PVP, result-ing in a more hydrophobic polymer chain. Thus,consistent with our observations, weak to no bindingoccurs upon the addition of citrate, sulfate, hydrogenphosphate, and acetate. In contrast, the anions listed tothe right of chloride are classified as chaotropes. Theirhydrophobic character disrupts water-water interactions,allowing them to favorably interact with the hydrophobicsurface of PVP. Of the chaotropes listed, PVP binds nitrite,nitrate, iodide, and thiocyanate but does not bind bro-mide or chlorate. We note that our observations are in line

Figure 2. (a) Fractional quenching (Ff/F0) and (b) apparentdissociation constants (Kd) of 1 wt% PVP40 to anions (Ff= 400 mM, F0 = 0 mM) as a function of pH in 50 mM sodiumcitrate buffer (pH 4, black), 50 mM sodium citrate buffer (pH 6,dark grey), and 50 mM potassium phosphate buffer (pH 8,white). (c) Fractional quenching and (d) Kd values of 1 wt%PVP to anions as a function of molecular weight in 50 mMsodium citrate buffer (pH 6) with K15 (average MW = 10,000 g/mol, black), PVP40 (average MW = 40,000 g/mol, grey), and K90(average MW = 360,000 g/mol, white). All spectra and bindingcurves are shown in Figures S3-S6 and Tables S1 and S2.

SUPRAMOLECULAR CHEMISTRY 5

with that previously reported using halide NMR spectro-scopy (SO4

2- < F− < Cl− < NO3− < Br− < I− < SCN−) with the

exception of bromide. These deviations could arise fromdifferences in how the experimental methods probe theelectronic structure of PVP in the presence of anions. Fromthis reasoning, anion binding to PVP could displace watermolecules and affect its spectroscopic properties.

Next, we carried out MD simulations to understandthe interactions of anions with PVP in water. The anionstested included sulfate, chloride, bromide, nitrite,nitrate, iodide, and thiocyanate. Enhanced anion pene-tration within 15 Å of PVP is observed for nitrite, nitrate,iodide, and thiocyanate in the MD trajectories (Figure 3and S9). No atoms are found within ~2 Å of PVP due toexcluded volume and interestingly, no sulfate ions arefound within ~8 Å of PVP (Figure 4(b)). The radial dis-tribution functions (RDFs) for nitrate, bromide, iodide,and thiocyanate display peaks and troughs indicative ofanion structuring around PVP whereas the others donot (Figure 4(b) and S10). This structuring of anions canbe arranged as Br− < SCN− < NO3

− < I− according to thedistance at which their maximum probabilities areobserved around the nitrogen and carbon atoms inthe lactam bond of PVP (Figure 4(b) and S10). A three-dimensional distribution analysis shows distinct regionswhere the anions are localized around the PVP mono-mer (Figure 4(a) and Table S4). These regions can lar-gely be attributed to the electrostatics of each PVPmonomer and its adjacent monomers in the polymerchain (Figures 4(a) and S8). We also counted the num-ber of anions in direct contact with the PVP surface tomonitor the ion adsorption. This information isobtained from the integral of the RDF (Figure 3(c))where we define a contacting ion to be within ~5.0 Å(roughly the excluded volume plus one atomic dia-meter) of PVP. Using this approach, the anions areranked as follows: NO2

− > NO3− > SCN− > I− > Br− >

Cl− > SO42-.

Furthermore, RDFs for other species were computedto explain the reorganization of water molecules and tojustify the above results (Figure S11). Here, we highlightthe most noteworthy differences. Compared to otheranions, water is found with the highest probabilityaround chloride. This indicates a strong hydrationshell around chloride corresponding to its low distribu-tion around PVP (Figure S11A). Sodium ions are clearlystructured into two solvation shells around PVP whichstabilizes the anion ordering in all cases except forsulfate (Figure S11D); the strong sulfate-sulfate andsodium-sulfate ordering effectively segregates sulfatefrom PVP as previously reported for other amide con-taining polymers and model peptides (Figures S11B andS11C) (60, 61). Finally, we can monitor how aggregated

the PVP structure is by measuring its solvent accessiblesurface area (SASA), where lower exposed PVP surfacearea means fewer possible interaction sites for anionadsorption (Figure S12). The systems can be arranged indecreasing order of SASA as follows: NO3

− > I− > H2O >SCN− > NO2

− > SO42- > Br− > Cl−. This analysis provides

a possible explanation for the observed anion-inducedfluorescence quenching. A larger SASA value correlateswith less aggregation, hence the fluorescence of PVPwould be quenched if it arises from AIE. Additional

Figure 3. Snapshots of 200 ns MD simulations of PVP in (a)water without ions, (b) sulfate, (c) chloride, (d) nitrite, (e)nitrate, (f) bromide, (g) iodide, and (h) thiocyanate in water.Coloring scheme is as follows: PVP (green), water (transparentblue), sodium (pink), nitrogen (blue), oxygen (red), sulfur (yel-low), chloride (cyan), bromide (yellow-lime), and iodide(magenta).

6 H. C. KAM ET AL.

experiments would be required to confirm the quench-ing mechanism, but these are beyond the scope of thisstudy. Overall, the MD simulations show good agree-ment with our experimental studies by providinga molecular level picture of individual contributing fac-tors. The set of anions found closest to PVP are found to

quench the fluorescence of PVP. Moreover, the largehydration shells around sulfate and chloride both cor-relate with their minimal interaction with PVP and thelack of fluorescence quenching.

In closing, here we show the expanded selectivityand in silico modelling of off-the-shelf PVP as a turn-offfluorescent polymer for nitrite, nitrate, iodide, and thio-cyanate in aqueous media. Several conclusions can bedrawn from this study as follows. First, since PVP lacksany hydrolysable bonds between pH 4 to 8, the anionselectivity is unaffected by pH but anion affinity to PVPis affected. Second, anion selectivity is not governed bythe chain length of PVP between 10,000 to 360,000average molecular weight. The fractional quenchingcan be ranked as SCN− < I− < NO3

− < NO2−. However,

the dynamic range and anion affinity can be fine-tunedby altering the chain length, hence the highest affinityfor nitrite is with K15, thiocyanate with PVP40, andnitrate and iodide with K90. Third, the fluorescencequenching of PVP deviates from the traditionalHofmeister series. In the Hofmeister series, kosmotropesare more solvated than chaotropes, favoring ion-ionand ion-water interactions. As expected, this is reflectedin the MD simulations showing longer contact distancesto PVP, lower SASA values, and no experimentallyobserved fluorescence quenching. Chaotropes, on theother hand, are expected to quench the fluorescence ofPVP. However, bromide and chlorate do not show mea-surable binding to PVP despite their chaotropic proper-ties. With bromide, the MD simulations show anordering in water, but it is more hydrated and displaysa greater contact distance to PVP, giving rise to lowerSASA values and no change in fluorescence. Giventhese exceptions, the data reflects the Hofmeister seriesbetween the two extremes with variability in directranking of fractional quenching and anion affinity. Weanticipate that MD simulations can be integrated intothe workflow to rationally design anion-responsivepolymers. Overall, this fundamental study not only con-tributes to the field of molecular recognition in waterbut also provides a starting point to confer anion bind-ing properties with a fluorescence output in new poly-olefin-based materials.

Acknowledgments

We thank Professor Gabriele Meloni, Dr. Koushambi Mitra, Ms.Whitney Ong, and Ms. Jasmine Tutol for helpful discussions.

Disclosure statement

No potential conflict of interest was reported by the authors.

Figure 4. (a) Spatial distribution of iodide ions (purple) aroundthe PVP repeating unit shown as carbon (cyan), oxygen (red),nitrogen (blue), and hydrogen (white). The oxygen atoms of theadjacent monomers (transparent yellow) and charge distribu-tion of the monomer oxygen (transparent blue) are shown. (b)Radial distribution functions (RDFs, g(r)) of water (black), sulfate(red), chloride (dark blue), nitrite (purple), nitrate (green), bro-mide (yellow), iodide (orange), and thiocyanate (light blue)versus radial distance (r). (c) Integrated radial distributions ofthe anions above versus radial distance. All RDFs are shownaround the labeled C and O atoms of PVP for nitrate and nitriterespectively, and around the N atom of PVP for all other anionstested.

SUPRAMOLECULAR CHEMISTRY 7

Funding

This work was supported by the Welch Foundation [AT-1918-20170325].

ORCID

Hiu C. Kam http://orcid.org/0000-0003-4047-9151Dineli T. S. Ranathunga http://orcid.org/0000-0002-7860-722XEthan R. Payne http://orcid.org/0000-0002-6620-8336Ronald A. Smaldone http://orcid.org/0000-0003-4560-7079Steven O. Nielsen http://orcid.org/0000-0003-3390-3313Sheel C. Dodani http://orcid.org/0000-0003-0271-6080

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SUPRAMOLECULAR CHEMISTRY 9