Electrochemical-Shock Synthesis of Nanoparticles from Sub-femtoliter Nanodroplets.

ConspectusNanoparticles have witnessed immense development in the past several decades due to their intriguing physicochemical properties. The modern chemist is interested not only in methods of synthesizing nanoparticles with tunable properties but also in the chemistry that nanoparticles can drive. While several methods exist to synthesize nanoparticles, it is often advantageous to put nanoparticles on a variety of conductive substrates for multiple applications (such as energy storage and conversion). Despite enjoying over 200 years of development, electrodeposition of nanoparticles suffers from a lack of control over nanoparticle size and morphology. There have been heroic efforts to address these issues over time. With an understanding that structure-function studies are imperative to understand the chemistry of nanoparticles, new methods are necessary to electrodeposit a variety of nanoparticles with control over macromorphology and also microstructure.This Account details our group's efforts in overcoming challenges of classical nanoparticle electrodeposition by electrodepositing nanoparticles from water nanodroplets. When a nanodroplet full of metal salt precursor is incident on the electrode biased sufficiently negative to drive electroplating, nanoparticles form at a fast rate (on the order of microseconds to milliseconds). We start with the general nuts-and-bolts of the experiment (nanodroplet formation and methods for electrodeposition). The deposition of new nanomaterials often requires one to develop new methods of measurement, and we detail new measurement tools for quantifying nanoparticle porosity and nanopore tortuosity within single nanoparticles. We achieve nanopore characterization by using Focused Ion Beam milling and Scanning Electron Microscopy. Owing to the small size of the nanodroplets and fast mass transfer (the contents of a femtoliter droplet can be electrolyzed in only a few milliseconds), the use of nanodroplets also allows the electrodeposition of high entropy alloy nanoparticles at room temperature.We detail how a deep understanding of ion transfer mechanisms can be used to expand the library of possible metals that can be deposited. Furthermore, simple ion changes in the dispersed droplet phase can decrease the cost per experiment by orders of magnitude. Finally, electrodeposition in aqueous nanodroplets can also be combined with stochastic electrochemistry for a variety of interesting studies. We detail the quantification of the growth kinetics of single nanoparticles in single aqueous nanodroplets. Nanodroplets can also be used as tiny reactors to trap only a few molecules of a metal salt precursor. Upon reduction to the zerovalent metal, electrocatalysis at very small metal clusters can be probed and evaluated with time using steady-state electrochemical measurements. Overall, this burgeoning synthetic tool is providing unexpected avenues of tunability of metal nanoparticles on conductive substrates.

The electrodeposition of gold nanoparticles from aqueous nanodroplets.

Nanodroplet-mediated electrodeposition is a reliable method for electrodepositing nanoparticles by confining a small amount of metal-salt precursor in water nanodroplets (radius ∼400 nm) suspended in an oil continuous phase. This technique provides a great advantage in terms of nanoparticle size, morphology, and porosity. For an electrochemical reaction to proceed in the aqueous nanodroplet, the electroneutrality condition must be maintained. Classically, [NB4][ClO4] or a comparable salt is added to the oil continuous phase to maintain charge balance. Unfortunately, the presence of this salt in the oil phase causes some metal salts, such as HAuCl4, to phase transfer, disallowing the formation of gold nanoparticles. Here, we demonstrate the partitioning of HAuCl4 is orders of magnitude lower using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) when LiClO4 is added to the nanodroplet phase and [NBu4][ClO4] is not added to the continuous phase. This simple change allows for the electrodeposition of gold nanoparticles. Scanning electron microscopy shows the morphology and size distribution of gold nanoparticles obtained at different concentrations of LiClO4. Transmission electron microscopy in selected diffraction mode was used and it determined the gold nanoparticles obtained are polycrystalline with miller indices of (222) and (200). This work widens the variety of nanoparticles that can be electrodeposited from nanodroplets for applications in energy storage and conversion, photoelectrochemistry, and biosensing.

Enabling practical nanoparticle electrodeposition from aqueous nanodroplets.

The rapid rise of technology in the modern world has led to an increased demand for energy. Consequently, it is essential to increase the efficiency of current energy-producing systems due to the poor activity of their catalysts. Nanoparticles play a significant role in energy storage and conversion; however, electrodeposition of nanoparticles is difficult to achieve due to surface heterogeneities, nanoparticle diffusion layer overlap, and the inability to electrodeposit multi-metallic nanoparticles with stoichiometric control. These problems can be solved through nanodroplet-mediated electrodeposition, a technique where water nanodroplets are filled with metal salt precursors that form stable nanoparticles when they collide with a negatively-biased electrode. Further, this method has demonstrated control over nanoparticle size and morphology, displaying a wide variety of applications for the generation of materials with excellent catalytic properties. Historically, the cost of nanodroplet-mediated electrodeposition experimentation is prohibitive because practitioners use 0.1 M to 0.5 M tetrabutylammonium perchlorate (TBAP) dissolved in the oil phase (∼10 mL). Such high concentrations of electrolytes have been used to lower ohmic drop and provide ions to maintain charge balance during electrodeposition. Here, we show that supporting electrolyte is not necessary for the oil phase. In fact, one can use a suitable salt (such as lithium perchlorate) in the aqueous phase to achieve nanoparticle electrodeposition. This simple change, grounded in an understanding of ion transfer, drives down the cost per experiment by nearly three orders of magnitude, representing a necessary step forward in enabling practical nanoparticle electrodeposition from water nanodroplets. This approach is a promising procedure for future cost-effective energy conversion systems relying on electrocatalytic nanoparticles.

Measurement of Neuropeptide Y Using Aptamer-Modified Microelectrodes by Electrochemical Impedance Spectroscopy.

Aptamer-modified microelectrodes for Neuropeptide Y measurement by electrochemical impedance spectroscopy was described here. The advantages of using carbon fiber or platinum microelectrodes are because they are promising materials with high electrical conductivity, chemical stability, and high surface area that can be easily modified on their surface. The immobilization and biofouling were studied and compared using EIS. Moreover, the adsorption of NPY to the aptamer-modified microelectrodes was also demonstrated by EIS. Changes of -ω*Zimag, an impedance factor that gives information of the capacitance, is directly correlated with concentrations. A widely linear range was obtained from 10 to 1000 ng/mL of NPY. This method was able to detect NPY without performing a redox reaction by adsorption at the surface of the microelectrodes, with the specificity provided by aptamer functionalization of the microelectrode surface.