Supporting Information

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1 Supporting Information Regulation of Interparticle Forces Reveals Controlled Aggregation in Charged Nanoparticles Anish Rao, Soumendu Roy, Mahima Unnikrishnan, Sumit S. Bhosale, Gayathri Devatha and Pramod P. Pillai* Department of Chemistry, Indian Institute of Science Education and Research (IISER), Dr. Homi Bhabha Road, Pune , Maharashtra, India *Correspondence to: Section 1: Experimental Details Materials and reagents: The positively charged N,N,N-trimethyl(11- mercaptoundecyl)ammonium ion (TMA) was synthesized according to the reported procedure. S1 Tetrachloroaurate trihydrate (HAuCl4.3H2O), Tetramethylammonium hydroxide (TMAOH) 25 % wt. in water, 11-mercaptoundecanoic acid (MUA), Hydrazine monohydrate (N2H4.H2O 50-60%), Sodium citrate tribasic dihydrate, Tetrabutylammonium borohydride (TBAB), divalent and monovalent salts- Ca(NO3)2.4H2O, Pb(NO3)2, NaNO3, Mg(NO3)2.6H2O and Cd(NO3)2.4H2O were purchased from Sigma Aldrich. (Di-n-dodecyl)dimethyl ammonium bromide (DDAB) and Dodecylamine (DDA), were purchased from Alfa Aesar. All the reagents were used as received without any further purification. All the stock solutions of metal ions were prepared in deionized water. Synthesis of [+/-]n AuNPs (Place exchange reaction) S2 : DDA-AuNPs (20 ml) were first purified by precipitating them in methanol (50 ml), yielding a black precipitate. The precipitate was then re-dissolved in 20 ml of toluene and a mixture of [+] and [-] (dissolved in 10 ml of S1

2 dichloromethane, DCM) was added to the toluene solution of DDA-AuNPs. The solution was kept undisturbed for ~ 15 h to equilibrate. Next, the supernatant solution was decanted and the precipitates were washed with dichloromethane (3 x 50 ml) followed by acetone (1 x 50 ml). The precipitate was then dried, re-dispersed in water, and 20 µl of TMAOH (25 % wt. in water) was added to deprotonate the carboxylic acid groups in [-]. Various ratios of [+] and [- ] (nsoln = [-] / [+] = 9, 4, 3 etc.) were used during the place exchange reaction. A ~40 fold molar excess of ligands were added during the place exchange to confirm the complete removal of DDA. The average size of the nanoparticles thus synthesized were 6.0 ± 0.7 nm, as estimated from TEM analysis (~ 350 NPs were counted for estimating the size distribution). Figure S1. Schematic representation of place exchange reaction. [+/-]n AuNPs are prepared by replacing DDA ligand with a mixture of [+] and [-] charged ligands. A similar experimental procedure was adopted for the preparation of [-] AuNPs, where, only [-] ligands were fed during the place exchange reaction. The [DDT/-]4 AuNPs were prepared by adding DDT:[-] ligands in the ratio 1:4 during the place exchange reaction. UV-Vis studies: Extinction spectra of NPs was recorded in Perkin Elmer Lambda 45 spectrophotometer in an optical glass cuvette (10 mm path length) over the entire visible range of nm. Concentration of the nanoparticles was taken in such a way that the optical density of the solution was ~ 0.25 (~ 5 nm in terms of AuNPs). The ph of the NP solution was ~ 7. S2

3 Dynamic Light Scattering (DLS) and Zeta potential (ζ) studies: The hydrodynamic diameter and zeta potential (ζ, at ph ~ 11 and ~ 7) of charged AuNPs were measured in Nano ZS90 (Malvern) Zetasizer instrument. The optical density of all the NP solutions was kept around ~ 0.2. ζ was determined by measuring the electrophoretic mobility and using Henry s equation U E = 2εzf(κ a) 3η Where, UE = electrophoretic mobility z ε η = zeta potential = dielectric constant = viscosity f(κa) = Henry s function Smoluchowski s approximation was used to measure the zeta potential values of NPs. The ζ values reported are based on an average of 3 measurements, and error was less than 1.5 mv in all the NP systems. Microscopy studies: We performed microscopy (TEM, AFM and SEM) experiments in order to characterize the size of the aggregates formed. Microscopic sample s preparation was carried out carefully to minimize the effect of sample drying on aggregate formation. In a typical experiment, an aqueous solution of [-] AuNP (5 nm in terms of AuNPs) was mixed with 1mM Pb 2+ and the resulting solution was left undisturbed for 3h (The aggregate formation was confirmed by monitoring the bathochromic shift). Then, ~100 L of [-] AuNP-Pb 2+ aggregate solution was drop casted on different substrates (Formvar coated Cu grid for TEM, mica substrate for AFM and Si wafer for SEM). The drop was subsequently removed by the means of a tissue paper after ~10 min so as to minimize the drying effect. The samples were dried S3

4 at room temperature before imaging. The AFM imaging was performed on Key Sight 5500 instrument (Agilent Technologies) under tapping mode with silicon nitride tip. Trapping experiments: Trapping experiments were performed with different AuNP ratios (differing in the magnitudes of surface charges) in the presence of varying concentrations of Pb 2+. In a typical trapping experiment, aqueous solution of [-] AuNP (3 ml) was prepared such that the optical density was ~0.25 (~ 5 nm in terms of AuNPs), followed by the addition of 1mM of Pb 2+. The progress of trapping was monitored by UV-Vis extinction, DLS and microscopy (TEM, AFM and SEM) studies. Spectral changes were followed by monitoring UV- Vis extinction spectrum for at least 1 day. DLS measurements were performed for ~3h so as to gain insight on the variation of hydrodynamic diameter of AuNPs with time. Microscopy experiments were carried out by drop casting ~ 100 L of AuNP-Pb 2+ aggregate solution (after 3h of incubation in Pb 2+ ) on the respective substrates. The amount of Pb 2+ trapped by AuNPs (after removing the un-coordinated Pb 2+ by dialysis) was determined by ICP-MS (Thermo Scientific, ELEMENT XR TM ICP-MS). Scavenging and quantification of trapped Pb 2+ : In a typical experiment, Pb 2+ were added to a 3 ml solution of 5 nm [+/-]4 AuNPs (in terms of AuNPs). The volume of Pb 2+ was adjusted to achieve a final concentration of 35 mm in 3 ml solution of [+/-]4 AuNPs. After 3h, the solution was kept for dialysis (in a 2 kda dialysis bag) for 1day. This ensured the complete removal of un-coordinated Pb 2+ from the system. Then, 1 ml of aqua regia (3:1 v/v HCl: HNO3) was added to the NP solution inside the dialysis bag to digest the AuNPs. The amount of Pb 2+ in the digested sample were quantified using ICP-MS analysis performed on ELEMENT XR ICP-MS (Thermo Scientific) instrument. Proper blank experiments in the absence of Pb 2+ were conducted and were taken as the baseline. The average of three different measurements was taken to calculate the amount of trapped Pb 2+. S4

5 Section 2: Characterization of charged NPs Figure S2. Extinction and TEM studies of charged AuNPs. (a) Extinction spectra of [-] AuNP and [+/-]4 AuNP. (b) TEM images and size distribution of 6 ± 0.7 nm [+/-]4 AuNPs. The size distribution was estimated from ~ 350 NPs. 30 Zeta potential (mv) [-] [+/-] 9 [+/-] 4 [+/-] 3 [+/-] 1 [+/-] 0.25 [+] [DDT/-] 4 AuNPs Figure S3. Zeta potential ( ) studies of charged AuNPs. Plot showing the variation of ζ potential with m-sam composition at ph ~ 11. The error bars correspond to standard deviations based on three different sets of experiments. Section 3: Optimization of controlled aggregation phenomena in charged AuNPs. S5

6 Figure S4. Screening of charged AuNPs for finding out the stable AuNP-Pb 2+ aggregate system. Extinction studies of charged AuNPs in the presence of varying amount of Pb 2+. Details about the AuNPs and Pb 2+ are given in the respective figures Figure S5. Extinction studies of [+/-]4 AuNP in 100 mm Pb 2+. (a, b) Time dependent extinction spectral changes in [+/-]4 AuNP in 100 mm Pb 2+. A bathochromic shift of ~ 20 nm was observed and the NPs precipitate after ~ 7 h. This by itself was interesting as the NP aggregates in 100 mm Pb 2+ were stable for an appreciable period, providing the time to scavenge them out of the system. S6

7 Section 4: Microscopic characterization Figure S6. TEM images of [-] AuNP-Pb 2+ aggregates. TEM images showing the formation of large aggregates (precipitates) of [-] AuNP upon interaction with 1 mm Pb 2+. S7

8 Figure S7. TEM images of [+/-]4 AuNP-Pb 2+ aggregates. TEM images showing the formation of controlled aggregates of [+/-]4 AuNP upon interaction with 35 mm Pb 2+. S8

9 Figure S8. Tapping mode AFM images of [-] AuNP-Pb 2+ aggregates. AFM (a), height and (b), phase images showing the formation of large aggregates (precipitates) of [-] AuNP upon interaction with 1 mm Pb 2+. S9

10 Figure S9. Tapping mode AFM studies of [+/-]4 AuNP-Pb 2+ aggregates. This is the full area image of the AFM height image already shown in the main text. The AFM height image confirmed the formation of smaller and controlled aggregates of [+/-]4 AuNP upon interaction with 35 mm Pb 2+. S10

11 Figure S10. Tapping mode AFM studies of [+/-]4 AuNP-Pb 2+ aggregates. This is an enlarged version of the AFM images already shown in the main text. (a), The 3D and (b), 2D AFM height images showing the boundaries between individual NPs forming the aggregates between [+/-]4 AuNP and 35mM Pb 2+ (ripples and bright lines in (a) and (b), respectively). S11

12 Figure S11. Tapping mode AFM studies of [+/-]4 AuNP-Pb 2+ aggregates. This is an enlarged version of the AFM image already shown in the main text. An AFM phase image showing the boundaries between individual NPs forming the aggregates between [+/-]4 AuNP and 35mM Pb 2+ (dark lines in between NPs in the aggregates). Figure S12. SEM images of [-] AuNP-Pb 2+ aggregates. SEM images showing the formation of large aggregates (precipitates) of [-] AuNP upon interaction with 1 mm Pb 2+. S12

13 Figure S13. SEM images of [+/-]4 AuNP-Pb 2+ aggregates. SEM images showing the formation of many small aggregates (<10 particle aggregates) of [+/-]4 AuNP upon interaction with 35 mm Pb 2+. S13

14 Section 5: Mechanism of interaction in NP- Pb 2+ system NMR Studies: Before performing NMR investigation, gold cores of the NPs were etched with molecular I2, using the reported literature procedure. S4 Excess of I2 was removed by washing with methanol and drying at 65 o C. The thiol mixtures were then dried under vacuum for ~15 h to remove the traces of water and methanol. The purified thiol mixtures were dissolved in deuterated DMSO and 1 H NMR spectrum was taken on a 400 MHz Bruker apparatus. A typical spectrum of [-]/[+] thiol solution (αsoln = n=4) in d 6 -DMSO is shown in Figure S17. The composition of the mixture was estimated as follows:- [ ] surf Mixed peak [+]surf = [+] surf [+] surf (1) = ([+]surf + [ ] surf ) [+] surf [+] surf The mixed peak corresponds to methylene protons next to thiol (--CH2--S--) from both [+] and [-] ligands (at δ ~ 2.7 ppm). The αsurf was then calculated in the following way: All the peaks were integrated with respect to the reference peak at δ ~ 3.26 ppm (corresponding to (-CH2 N + (CH3)3). The integration values corresponding to the mixed peak ([+] + [-]) at δ ~ 2.69 ppm) and [+] peak at δ ~ 3.03 ppm (recalculated for single methyl proton in N + (CH3)3) were substituted in equation (1) to get αsurf to be 3.0. The αsurf value obtained here is in accordance with the literature values. S2 S14

15 Figure S14. 1 H NMR spectrum of [-]/[+] thiol solution (αsoln = n=4) in d 6 -DMSO in d 6 -DMSO after etching the Au cores. Figure S15. Interaction of Pb 2+ with [+] AuNPs. The negligible change ( max ~ 1 nm) is observed in the extinction spectrum of [+] AuNPs upon addition of 35 mm Pb 2+. S15

16 Figure S16. Interaction of Pb 2+ with [+/-]4 AuNP at acidic ph. The insignificant change ( max ~ 2 nm) witnessed in the extinction spectrum after the addition of 35mM of Pb 2+ to an acidic solution of [+/-]4 AuNPs. This validates the necessity of carboxylate group at the NP surface for interaction with Pb 2+. The control Pb 2+ binding experiments carried out with [+] Au NPs and [+/-]4 AuNPs (at acidic ph) substantiate that the aggregation of NPs occur through bridging interactions and not by the desorption of thiol molecules. S16

17 Figure S17. Effect of addition of Pb 2+ on Zeta Potential of [+/-]4 AuNPs. The ζ plots for [+/-]4 AuNP in the (a) absence and (b) presence of 35 mm Pb 2+. The negative ζ value indicates the presence of excess negatively charged ligands on the surface of untreated [+/-]4 AuNPs. Interestingly, there is a reversal in ζ value (from -8 ± 1.2 mv to 14 ± 15 mv) upon addition of Pb 2+ which confirms that the stability in [+/-]4 AuNP-Pb 2+ is imparted by positively charged [+] ligands in the m-sam. S17

18 Section 6: Reversibility Studies The reversibility experiments were carried out with [+/-]9 AuNP in 1mM Pb 2+, since the spectral responses were faster and [+/-]9 AuNP-Pb 2+ aggregates were stable for an appreciable time (~ 7 h). Ethylenediaminetetraacetic acid (EDTA) is one of the most commonly used reagents to realize the reversibility in COO - Pb 2+ trapping systems because of its extraordinary chelation ability. However, the previous studies and ours have shown that the use of EDTA results in a partial release of Pb 2+, and small aggregates still persist in the system. Moreover, the EDTA can also interact with the [+] charges on the m-sam of [+/-] AuNPs. This led to the search of alternate routes for achieving the reversibility, and we came up with simple acid-base chemistry. We used NaOH (1M) and HCl (1M) to disassemble and reassemble the NP-ion aggregates, respectively. Figure S18. Assembly-disassembly-reassembly studies in [+/-] AuNP-Pb 2+ system. Variation in extinction spectra of [+/-] AuNPs upon (a) assembly-disassembly and (b) reassembly processes. The assembly and disassembly processes were carried out by the addition of Pb 2+ and OH ions, respectively. The reassembly process was carried out by the addition of HCl. Assembly and Disassembly: A solution of [+/-]9 AuNP containing 1 mm Pb 2+ (as described above) was prepared. Once the aggregation was confirmed (after monitoring the bathochromic shift, red spectrum in Supplementary Fig 8a), 10 ul quantities of 1 M NaOH was added to see the disassembling process. Upon addition of NaOH, the solution starts to turn turbid without S18

19 change in the λmax. The turbidity is due to the formation of insoluble Pb(OH)2 (NaOH reacts with the excess Pb 2+ in the solution). On further additions of NaOH, the turbidity decreased because of the formation of soluble plumbate (PbO2 2- ). Once the solution becomes completely clear, the addition of NaOH leads to a blue shift in the λmax indicating the disassembly of the controlled aggregates (blue spectrum in the Supplementary Fig 8a). Reassembly: To reassemble the system, 1M HCl was added to the above system. Addition of HCl dissociates plumbate and lead hydroxide rendering Pb 2+ free (which can reassemble the system giving controlled aggregates). The initial additions of 1M HCl neutralizes the excess of base present in the solution because of which, one does not observe changes in the extinction spectrum. Further addition of acid dissociates the PbO2 2- to Pb(OH)2 and ultimately releases the Pb 2+ by dissociating the Pb(OH)2. The free Pb 2+ then chelates with the carboxyl groups resulting in the re-assembly of NPs. In order to establish the role of Pb 2+ during the reassembly process, control experiments involving the same amounts (as has been used above for disassembly and the reassembly) of - OH and H + were added to [+/-]9 AuNP in the absence of Pb 2+. We followed the spectral changes of the resulting solution with each addition and found no shift in max showing the need of Pb 2+ during the assembly process The chemical reactions taking part in the disassembly process are described below: S19

20 Figure S19. Schematic representation of the chemical changes occurring during a single assembly disassembly cycle. This is an enlarged version of the scheme (with additional details) which is already shown in Figure 6d of the main text. Monodisperse [+/-] AuNPs forms stable aggregates upon interaction with Pb 2+. Addition of OH disassembles the aggregates by removing the trapped Pb 2+ to form insoluble Pb(OH)2. Further addition of OH converts the insoluble Pb(OH)2 to soluble plumbate (PbO2 2- ). Addition of H + then leads to the re-trapping of Pb 2+, by first neutralising the excess OH and then dissociating PbO2 2- to free Pb 2+. The reactions taking part during the whole assembly-disassembly process are shown (below the scheme). Formation and Dissociation of plumbate: It is known that upon addition of sodium hydroxide to a solution of lead (II) nitrate, a white precipitate of lead (II) hydroxide is formed S5. The precipitate formed is responsible for the turbidity of the solution. The reaction for the step is as follows: S20

21 If excess sodium hydroxide is added, the precipitate re-dissolves to give a colorless solution. The compound formed is called plumbate (II) S5. The reaction is as follows: Figure S20. Reversibility experiment with [-] AuNP in 1mM Pb 2+. Extinction spectral changes in [-] AuNP-Pb 2+ upon the addition of 1 M NaOH. The decrease in the extinction intensity of [-] AuNP-Pb 2+ (purple curve) is due to the dilution effect caused by the addition of excess NaOH (~ 2 ml). S21

22 Section 7: Controlled aggregation phenomenon with Cd 2+, H + and citrate ions. Figure S21. Controlled aggregation in [+/-]4 AuNP with Cd 2+. Extinction spectral changes of [+/-]4 AuNPs in presence of 0.1 mm Cd 2+. A bathochromic shift of ~ 7 nm was observed without any compromise on the colloidal stability. Figure S22. Controlled aggregation in [+/-]9 AuNP with H +. Extinction spectral changes of [+/-]9 AuNPs in presence of 3.3 mm H +. A bathochromic shift of ~ 8 nm was observed without any compromise on the colloidal stability. S22

23 Figure S23. Controlled aggregation in [+/-]0.25 AuNP with citrate ions. (a), A schematic representation of citrate mediated bridging in [+/-]0.25 AuNPs. (b), Extinction spectral changes of [+/-]0.25 AuNPs and [+] AuNPs in presence of 0.1 mm citrate ions at ph ~ 11. A bathochromic shift of ~ 8 nm was observed in the case of [+/-]0.25 AuNPs without any compromise on the colloidal stability. Whereas, [+] AuNPs underwent a collateral decrease in the extinction and finally sedimented from solution. The citrate ions chelate with the positively charged quaternary ammonium groups initializing the aggregation phenomena. The [-] ligands on adjacent [+/-]0.25 AuNPs provided the necessary repulsions required to achieve the perfect balance between the interparticle forces, thus stabilizing the aggregates. Interestingly, the plasmon band shifts back to its initial position after ~ 3 h suggesting the release of citrate ions from the aggregates. A detailed investigation is required to get more insight into this interesting observation. References: S1. Tien, J.; Terfort, A.; Whitesides, G. M. Microfabrication Through Electrostatic Self- Assembly. Langmuir 1997, 13, S23

24 S2. Pillai, P. P.; Huda, S.; Kowalczyk, B.; Grzybowski, B. A. Controlled ph Stability and Adjustable Cellular Uptake of Mixed-Charge Nanoparticles. J. Am. Chem. Soc. 2013, 135, S3. Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Dynamics of Place-Exchange Reactions on Monolayer-Protected Gold Cluster Molecules. Langmuir 1999, 15, S4. Vogel, A. I. Vogel s Textbook of Macro and Semimicro Qualitative Inorganic Analysis Ch. 3 (Longman Inc., New York, 1979). S24