Interconnectivity between Surface Reactivity and Self‐Assembly of Kemp Elimination Catalyzing Nanorods

Abstract

Understanding the fundamental facts behind dynamicity of catalytic processes has been a longstanding quest across disciplines. Herein, we report self-assembly of catalytically active gold nanorods that can be regulated by tuning its reactivity towards a proton transfer reaction at different pH. Unlike substrate-induced templating and cooperativity, the enhanced aggregation rate is due to alteration of catalytic surface charge only during reactivity as negatively charged transition state of reactant (5-nitrobenzisoxazole) is formed on positively charged nanorod while undergoing a concerted E2-pathway. Herein, enhanced diffusivity during catalytic processes might also act as an additional contributing factor. Furthermore, we have also shown that nanosized hydrophobic cavities of clustered nanorods can also efficiently accelerate the rate of an aromatic nucleophilic substitution reaction, which also demonstrates a catalytic phenomenon that can lead to cascading of other reactions where substrates and products of the starting reactions are not directly involved.

Dictating Catalytic Preference and Activity of a Nanoparticle by Modulating Its Multivalent Engagement

“…It has been shown in earlier literature reports that the Kemp elimination (KE) reaction can be catalyzed in cationic miceller or vesicular media in the presence of a nucleophilic anionic base . Recently, we have also demonstrated that CTAB-functionalized GNP can accelerate the KE reaction by more than 2 orders of magnitude in the presence of phosphate buffer, where phosphate ion acts as the base on the surface of the GNP . Interestingly, the immediate surface environment of the cationic nanoparticle can be modulated by relying on self-assembly of anionic peptides or phosphates, which has been discussed in previous literature. , In fact, this strategy has been utilized for the development of innovative biosensors and catalysts for one particular kind of reaction (like ester hydrolysis) using the self-assembled nanoarchitecture. , Herein, we argued that the surface environment of the CTAB-functionalized cationic GNP can be altered by using a similar type of self-assembly approach but by changing the number of the phosphate units, so that the GNP can make itself host for multiple reactions .…”

“…However, a slight loss of colloidal stability of cationic GNP at 0.3 mM of ATP can be attributed to a reduction of the ξ-value below 30 mV (Figure 2b, Figure S7, SI). 15,19 It also suggests the effectiveness of binding increased with increasing valency, that is, simultaneous interaction with multiple charged phosphates like ATP. 6 We have also measured the amount of nucleotides (when added at 0.2 mM concentration) that is bound on the surface of GNP.…”

“…14 Recently, we have also demonstrated that CTABfunctionalized GNP can accelerate the KE reaction by more than 2 orders of magnitude in the presence of phosphate buffer, where phosphate ion acts as the base on the surface of the GNP. 15 Interestingly, the immediate surface environment of the cationic nanoparticle can be modulated by relying on self-assembly of anionic peptides or phosphates, which has been discussed in previous literature. 11a,16 In fact, this strategy has been utilized for the development of innovative biosensors and catalysts for one particular kind of reaction (like ester hydrolysis) using the self-assembled nanoarchitecture.…”

“…After exploring the surface pH, we wanted to explore how these can affect the reactivity of different reactions. First, we have chosen the proton transfer reaction (by following the KE reaction) as recently we have shown this base-catalyzed E2 type reaction can be accelerated by cationic GNP in the presence of phosphate buffer . The progress of the reaction has been followed by the rate of formation of 2-cyano-4-nitrophenol (CNP) at 380 nm while using 5-nitrobenzisoxazole (NBI) as the substrate as reported in the literature. , Here also the rate of the reaction can be increased in the presence of only cationic GNP compared with only buffer as the reaction is taking place on the surface, which has a 1.5-unit higher pH.…”

“…First, we have chosen the proton transfer reaction (by following the KE reaction) as recently we have shown this base-catalyzed E2 type reaction can be accelerated by cationic GNP in the presence of phosphate buffer . The progress of the reaction has been followed by the rate of formation of 2-cyano-4-nitrophenol (CNP) at 380 nm while using 5-nitrobenzisoxazole (NBI) as the substrate as reported in the literature. , Here also the rate of the reaction can be increased in the presence of only cationic GNP compared with only buffer as the reaction is taking place on the surface, which has a 1.5-unit higher pH. Interestingly, addition of AMP on the cationic GNP system can enhance the proton transfer reaction at a consistent rate and almost 5- and 6-fold enhanced activity was observed in the presence of 0.2 and 0.3 mM AMP, respectively (Figure a,b, Figures S14–S18, SI)).…”