Use of Photoacids and Photobases To Control Dynamic Self

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The use of photoacids and photobases to control the dynamic selfassembly of gold nanoparticles in aqueous and non-aqueous solutions Anna Yucknovsky, Somen Mondal, Alex Burnstine-Townley, Mohammad Foqara, and Nadav Amdursky Nano Lett., Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 23, 2019

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The use of photoacids and photobases to control the dynamic selfassembly of gold nanoparticles in aqueous and non-aqueous solutions Anna Yucknovsky, Somen Mondal, Alex Burnstine-Townley, Mohammad Foqara, Nadav Amdursky* Schulich Faculty of Chemistry, Technion – Israel Institute of Technology, Haifa, 3200003, Israel *Corresponding author, e-mail: [email protected]

Abstract Dynamic self-assembly of nanoparticles (NPs) for the formation of aggregates takes place out of thermodynamic equilibrium, and is sustained by external energy supply. Herein, we present light energy driven dynamic self-assembly process of AuNPs, decorated with pH sensitive ligands. The process is being controlled by the use of photoacids or photobases that undergo excited state proton or hydroxide transfer, respectively, due to their large pKa change between their ground and excited electronic states. The unique design is underlined by record subsecond conversion rates between the assembled and disassembled AuNPs states, and the ability to control the process using only light of different wavelengths. Measurements in both aqueous and non-aqueous solutions resulted in different self-assembly mechanisms, hence showing the wide versatility of photoacids and photobases for dynamic processes.

Keywords: Self-assembly; Nanoparticles; Photoacids; Photobases

ACS Paragon Plus Environment

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Gold nanoparticles (AuNPs) and their assembled structures are highly attractive materials due to their stability, simplicity of production and surface functionalization capabilities, as well as unique optical properties, which arise from their nanoscale size.1,2 Various promising AuNPs applications, including drug delivery, surface-enhanced Raman spectroscopy and electronics, involve self-assembly processes of the NPs,3–8 which constitutes a transition from the dispersed state into an ordered crystalline-like phase, and can be induced by destabilization of the colloid suspension.9 The self-assembly process can be directed by manipulating attractive and repulsive non-covalent interactions between single particles, which can be achieved either by introducing an intrinsically responsive mechanism to an external fields or by decorating the NPs with chemically functionalized ligands.7,9,10 Self-assembly is not limited to only static formation, as dynamic processes driven out of equilibrium by continuous energy supply can be rendered.11–13 This dynamic nature is also in the heart of many nanotechnological innovations that aim to develop bio-inspired smart systems. Light is a convenient way to deliver energy remotely, by adjusting delivered wavelength, intensity and location.14 Indeed, light has been used to control the dynamic self-assembly of NPs, and usually by the photoisomerization of azobenzene or spiropyran functionalized NPs.15–21 One of the main contributors in this field is Klajn and coworkers, where they addressed important aspects of dynamic self-assembly such as reversibility, stimuli, solvents and ligand diversity.22–26 They further showed that the advanced surface functionalization of AuNPs can be replaced by a light responsive medium containing spiropyran or its water soluble derivative.27,28 In their system, carboxyl functionalized AuNPs aggregation was controlled by the concentration of protons in the medium, in which merocyanin (open-ring state) acted as a base and captured protons, and upon photoisomerization, spiropyran (closed-ring state) was formed, resulting in the proton release. Due to this proton release upon light activation of the molecule, spiropyran has been termed a photoacid. However, spiropyran mediated proton dissociation is not directly induced by light, but rather is subordinate to photoisomerization, making its timescale slow relative to other photoacids.29,30 Another important aspect of a reversible system design is the pKa gap between the proton donor and the carboxyl functionalized AuNPs surface, and between the latter and a proton acceptor. With this in mind, our motivation was to develop photochromic medium, characterized by fast response rates and expanded pKa gaps. In our work, we are using a different type of photoacids and photobases, consisting of aromatic molecules that display properties of weak acids or bases in their ground electronic state, but exhibit great pKa drop or increase, respectively, in their excited state.31 As in previous studies,27,28,32–35 we decorated our AuNPs with carboxylic acid terminated thiols to control the dynamic of the self-assembly process. However, unlike previous studies, where the dynamicity of the self-assembly process was very slow, and it could take hours to go from one state to another, we show here a fast subsecond control of the self-assembly that takes place immediately after excitation. We achieve this by adding both a photoacid and a photobase in an aqueous environment, with different wavelengths of excitation; in this way, a significant pKa gap arises simply by changing the wavelength of excitation. We further show dynamic self-assembly of the AuNPs in nonaqueous environment, while discussing different mechanisms taking place between an excitedstate proton to hydroxide transfer, and the effect of the molecular charge. Measurements in aqueous solution: To begin with, we synthesized 3.8 nm AuNPs (Figure S1), decorated with 6-mercaptohexanoic acid (MHA) monolayer. The color of solution was red, with corresponding absorption band at 513 nm (Figure 1a), originating from the local surface plasmonic resonance (LSPR) effect, correlating to the AuNP size.1 We used UV-Vis


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spectroscopy to follow the self-assembly of our carboxylic acid-terminated AuNPs, driven by hydrogen bonding between the COOH groups to induce aggregation of the NPs. Indeed, upon titration of the AuNPs with HCl, a red shift of the LSPR peak took place as well an increase in the absorption in the red edge of the spectrum (Figure 1a), indicating AuNP aggregation. To control this process with light, we chose to use water-soluble pyranine (HPTS) as a photoacid and 6-methoxyquinoline (6MQ) as a photobase, each capable of excited state proton transfer and proton abstraction, respectively. Figure 1c and 1d show the UV-Vis absorption of HPTS and 6MQ, respectively, exhibiting different lowest energy absorption of the protonated HPTS (at 405 nm) compared to 6MQ (at 326 nm), and also the change in the absorption spectrum upon the deprotonation of HPTS or the protonation of 6MQ. The hallmark of photoacids and photobases is their different pKa values between their ground and excited states, in which HPTS has a ground-state pKa of 7.4 and an excited-state pKa* of 0.4, while the ones of 6MQ are pKa = 5.18 and pKa* = 11.8.29 The AuNPs aqueous solution was kept at pH 4.5, which is in the range of the approximate pKa value of the carboxylates,36 meaning that the MHA monolayer is partially deprotonated, preserving negative charge of the AuNP surface and maintaining metastable colloid suspension. At this pH (4.5), HPTS is protonated while 6MQ is deprotonated (to some extent), which enables proton release by HPTS and proton capture by 6MQ upon light excitation. Our main working hypothesis is that light excitation of the photoacids in solution will induce their proton dissociation, which in turn will induce the aggregation of the AuNPs, in a similar manner to the titration with HCl. To test it, we followed the UV-Vis absorption of our AuNPs solution with low-concentrated HPTS (Figure 1b), which allowed us to observe the LSPR peak, as the deprotonated HPTS peak will overlap the LSPR peak in high HPTS concentrations. Indeed, upon light excitation of HPTS, we observed similar changes in the LSPR peak position and an increase in the absorption at 800 nm as we observed with HCl titration (Figure 1a), which is an indication for AuNPs destabilization. AuNPs with no HCl added AuNPs HCl added

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Therefore, the MUA provides further steric stabilization to the AuNPs, which results in dispersed NPs in the non-aqueous solution. Due to the strong, broad absorbance of the MGCB cation peak at 620 nm, we could not use UV-Vis to follow the LSPR peak nor the absorption at 800 nm, related to AuNPs aggregation. Accordingly, we used DLS to follow the reversible self-assembly of the AuNPs (Figure 3c and 3d). Interestingly, and unexpectedly, we observed that while both MGCB and MGCB+ can be used to control the dynamic aggregation of the AuNPs by light, the mechanism is different for the two. For MGCB (Figure 3c), we found that the hydroxide release from the photobase by UV excitation prompted the aggregation of the AuNPs, with average solvodynamic radii changing from 10 to 150 nm. This process is reversible, where the aggregates re-dispersed 20 min after light excitation; and this cycle can be repeated many times. MGCB+, on the other hand, induced the immediate flocculation of the AuNPs even before the UV irradiation (Figure 3d), most probably due to MGCB+ OHdissociation that leads to some fraction of MGCB2+. Upon UV excitation of MGCB+, a remarkable AuNPs cluster growth from 100 nm to 500 nm was observed. Importantly, the dynamic cycle in the presence of MGCB+ was faster than with MGCB, and the large AuNPs clusters dissipated and reached their initial state only 5 min after excitation. We further used transmission electron microscopy (TEM) to follow the aggregation of the AuNPs in methanol solution containing MGCB, where dispersed colloidal AuNPs were observed before illumination, while aggregations patterns were observed after illumination (Figures 4 and S3, for higher and lower magnification, respectively). The observed aggregation pattern size in the TEM images (approximately 0.1 K Figure S3b) showed correlation with the measured size by DLS (Figure 3c). Interestingly, we observed that the large AuNPs aggregates that were formed following irradiation have larger inter-nanoparticle gaps on their perimeter (Figure 4b). Probably, this is due to the fast dynamicity of our system, which might led to some redispersion of the AuNPs during the TEM sample preparation time.

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Figure 3. UV-Vis absorption of (a) MGCB and (b) MGCB+ in basic and acidic methanolic solutions, showing their triarylmethane hydroxide and triarylmethane cation states, respectively. Insets: UV-Vis absorption of (a) MGCB and (b) MGCB+ before light irradiation (black curve), instantly after light irradiation (red curve), confirming hydroxide release, and 30 min after light irradiation (blue curve), showing recombination. (c), (d), and (e), DLS of Au-NPs-MGCB, -MGCB+ and –HPTS systems, respectively, in methanol. c - cycle, t - min after (c and d) 285 nm or (e) 405 nm light excitation.

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Figure 4. TEM images of AuNPs-MGCB solution before (a) and after (b) uv light illumination

In our final system here, we targeted the possibility of controlling the self-assembly of AuNPs in non-aqueous solution using light induced positive charge (proton) transfer by the use of HPTS. Interestingly, our DLS measurements (Figure 3e) showed that the light induced HPTS deprotonation resulted in the AuNPs dispersion and not in their aggregation as in aqueous solution. Based on our DLS measurements, we could differentiate between the following events. 1) Initially dissolved AuNPs underwent immediate aggregation upon first light excitation. 2) These aggregates were not stable and within 7 min after excitation, a spontaneous clusters growth took place, where the average solvodynamic diameter grew from 200 nm to 400 nm. 3) Additional excitation resulted in the AuNPs clusters returning to their initial aggregative state. Hence, the dynamic reversible transition is between two different aggregation states (Figure 3e). To explain our results, we should take into consideration both the attractive and repulsive interparticle forces. Electric repulsion, induced by AuNPs surface charge, and steric stabilization, mediated by long-chain MUA ligands, counteract the attractive effect of Van der Waals forces. Self-assembly can be enhanced by hydrogen bonding between MUA ligands and by the addition of ions (the light induced hydroxide or proton release as in our case), which considerably weaken electrostatic repulsion.39 Interestingly, our systems with MGCB, MGCB+ or HPTS displayed different self-assembly mechanisms. First, photobase and photoacid excitation showed opposite effects on the AuNPs self-assembly, leading to aggregation and dispersion, respectively. Another difference is related to the extent of aggregation in MGCB/+ systems. To explore these differences, we measured the L potential of the methanolic AuNPs solutions before and after light excitation (Figure 5). The initial L potential was -40, -14 and 30 mV for the MGCB-, MGCB+- and HPTS-AuNPs systems, respectively. The negative values are probably due to certain fraction of deprotonated MUA ligands on the AuNPs surface. Lower absolute value of L potential in MGCB+-AuNPs system is consistent with DLS measurements, indicating unstable colloid solution, resulting from the dissociation of MGCB+. In both MGCB/+-AuNPs systems we observed a similar change in the L potential upon light excitation, which we attribute to AuNPs aggregation (Figure 5a and 5b, red curves). Upon recombination,


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initial L potentials have been restored in both systems, indicating AuNPs dispersion (Figure 5a

and 5b, blue curves). In HPTS-AuNPs system, the L potential shifts from -30 to + 10 mV upon light excitation, probably due to the protonation of MUA ligands (Figure 5c, red curve).27

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Figure 5. Zeta potential of AuNPs in non-aqueous systems containing (a) MGCB, (b) MGCB+ or (c) HPTS, before light irradiation (black curve), instantly after light irradiation (red curve), and several minutes after light irradiation (blue curve).

We suggest the following mechanisms underlying self-assembly of AuNPs in non-aqueous systems. MGCB-AuNPs system (Scheme 2a): (1) prior to light excitation, the AuNPs are electro-sterically stabilized due to the well-soluble MUA ligands, carrying some negative charge density. 07N(2 Upon light induced hydroxide release, ionic strength of the solution increases, resulting in a thinner electric double layer of the AuNPs and weakening the repulsion between them,39 which results in aggregation. 0(N72 In the dark, MGCB recombines, allowing AuNPs aggregates to dissipate. MGCB+-AuNPs system (Scheme 2b): (1) prior to light irradiation, the AuNPs are destabilized due to some MGCB+ OH-dissociation, hence a fraction of free MGCB2+. 07N(2 As in the previous system, upon light induced hydroxide release, the repulsion forces between AuNPs are weakened, but more intensively due to the additional charge, resulting in more pronounced aggregation. 0(N72 In the dark, reverse processes take place. HPTS-AuNPs system (Scheme 2c): (1) As before, the MUA ligands provide both steric stabilization and introduce certain negative charge density to the AuNPs surface. 07N(2 Upon first light induced proton release by HPTS, the MUA ligands are protonated, resulting in positively charged AuNPs surface. 0(N'2 In the dark, HPTS recombines by a proton back transfer from MUA to HPTS, resulting in the appearance of electrically neutral AuNPs. Accordingly, hydrogen bridges between adjacent ligands can be formed, which induces AuNPs aggregation. 0'N(2 Upon repeating light irradiation, HPTS to MUA proton transfer takes place, resulting in AuNPs electric double layer restoration and subsequent hydrogen bonds breakage, leading to AuNPs dispersion.


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Acknowledgments S.M thanks the Grand Technion Energy Program (GTEP) post-doctoral fellowship and the PBC fellowship of Israel’s council of higher education for financial support. N.A thanks the Chaya Career Advancement Chair, the Russel Berrie Nanotechnology Institute (RBNI) and GTEP for their support. We thank A. Binder for the design and manufacture of the LEDs setup, and R. Nandi and Y. Agam for consultation.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Materials and Methods, TEM images of the AuNP’s, and UV-Vis absorption spectrum of the AuNP’s in the presence of HPTS and 6MQ at different cycles.

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Nano Letters

TOC graphic 269x171mm (300 x 300 DPI)


Use of Photoacids and Photobases To Control Dynamic Self

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