Richard Zare, Postdoctoral Faculty Sponsor
Optimizing Coaxial Sonic Spray Geometry for Generating Water Microdroplets.
Sonic spray creates a stream of neutral and charged microdroplets without application of voltage, heating, laser irradiation, or corona discharge. The solvent of interest flows through an inner capillary (usually constructed of fused silica) that is surrounded by an outer stainless-steel tube through which a nebulizing gas flows under pressure. This technique has been widely used as the interface in mass spectrometric studies for chemical analysis and for understanding microdroplet chemistry. We have used light scattering to characterize the size distribution and density for water microdroplets as a function of several parameters, such as water quality, water flow rate, nebulizing gas pressure, and sonic sprayer geometry. We find that the size distribution of the microdroplets, which is critical to many applications, depends most sensitively on the distance between the inner and outer capillary outlets and the gas flow pressure. The best performance as measured by the smallness of the microdroplet diameters is obtained when the gas flow pressure is the highest and there is no separation distance, d, between the two capillary outlets. In addition, at d = 0 mm, the microdroplet diameter distribution is nearly independent of the water flow rate, indicating that studies under these conditions can be scaled up.
View details for DOI 10.1021/acs.analchem.1c05337
View details for PubMedID 35191692
Microdroplets can act as electrochemical cells.
The Journal of chemical physics
2022; 156 (5): 054705
A water microdroplet in air or oil typically possesses an electric double layer (EDL) from the preferential adsorption of surface-bound ions at the periphery. We present the calculations of the ion gradients within a microdroplet at equilibrium, including systems containing buffers and water autoionization. These ion gradients are used to calculate the potential energy stored within the microdroplet. We consider how this stored potential energy can be utilized to drive chemical reactions, much like an electrochemical cell. Effective voltages as high as 111 mV are found for microdroplets having a low surface charge density (0.01 ions per nm2). Two sources of potential energy are investigated: (1) the electrostatic energy of the EDL of the microdroplet and (2) shifts in other chemical equilibria coupled to the main reaction through the EDL. A particularly important example of the latter is water autoionization, wherein the reaction of interest causes a flattening of the [H+] gradient within the EDL, resulting in a net recombination of H+ and OH- throughout the microdroplet. Numerical calculations are performed using a continuum model consisting of a balance between the electromigration and diffusion of ions throughout the microdroplet. Our treatment accounts for the autoionization of water and any chemical equilibrium of buffers present. The results are presented for uncharged water microdroplets with low amounts of salts and simple buffers in them. However, the calculational method presented here can be applied to microdroplets of any net charge, composed of any solvent, containing ions of any valence, and containing complex mixtures of chemical equilibria.
View details for DOI 10.1063/5.0078281
View details for PubMedID 35135250
- What Role Does the Electric Double Layer Play in Redox Reactions at Planar Electrostatically Charged Insulating Surfaces? TOPICS IN CATALYSIS 2021
A Wireless Implantable Potentiostat for Programmable Electrochemical Drug Delivery
IEEE Biomedical Circuits and Systems (BIOCAS)
View details for DOI 10.1109/BioCAS49922.2021.9644991
Effects of Weak Electrolytes on Electric Double Layer Ion Distributions.
The journal of physical chemistry letters
Many common experimental systems have electric double layers containing weak electrolytes, including systems with buffers. The pH at the boundary of the diffuse layer is an important parameter for determining the physicochemical state of the system, including surface charge density. We show that the Boltzmann equilibrium relation can be used as an exact solution for weak electrolyte electric double layers. Using these results, we provide a closed-form relation for the maximum pH change in a buffered electric double layer, in terms of the boundary potential. Importantly, our results suggest that equilibrium electric double layer concepts developed for strong electrolytes can be expanded to include weak electrolytes.
View details for DOI 10.1021/acs.jpclett.0c02247
View details for PubMedID 32915583
CONCENTRATION GRADIENTS INSIDE MICRODROPLETS.
Micro total analysis systems : proceedings of the ... [Mu] TAS International Conference on Miniaturized Chemical and Biochemical Analysis Systems. [Mu] TAS (Conference)
2020; 2020: 212-213
Small water microdroplets in microfluidic systems have a high surface charge density resulting from charged surfactants. As a result, an electric double layer forms inside the droplet. Depletion of ions from the center of the droplet to form the double layer can shift the concentration of ions dramatically from that of the microdroplet precursor solution. Here we show numerical solutions to the Gouy-Chapman model in spherical coordinates. Some notable effects include: 1) large percentages of the microdroplet volume experience very large DC electric fields; 2) many ions get forced into a Stern layer giving dramatically different conditions from the bulk.
View details for PubMedID 34557061
View details for PubMedCentralID PMC8457253
Simple model for the electric field and spatial distribution of ions in a microdroplet.
The Journal of chemical physics
2020; 152 (18): 184702
It is well established that the chemistry in microdroplets has been found to be radically different from reactions in bulk, particularly in the case of water. It has also been established that there is a threshold size for microdroplets to behave differently than droplets near the 10 µm diameter range. We present a three-dimensional electrostatic treatment in the spirit of the Gouy-Chapman model for double layers at interfaces. Our treatment predicts a strong concentration of charged molecules toward the surface of the droplet. As the droplet size deceases, the majority of the volume of the liquid experiences a large DC electric field. Such electric fields are highly unusual in a conducting fluid such as water. We believe that this unique environment helps to explain the reaction rate acceleration and new chemistry that have been observed in microdroplets compared to bulk phase.
View details for DOI 10.1063/5.0006550
View details for PubMedID 32414270
On-demand drug release from polypyrrole nanoparticulate films
AMER CHEMICAL SOC. 2019
View details for Web of Science ID 000525055501160
Electrically controlled drug release using pH-sensitive polymer films.
2018; 10 (21): 10087–93
Drug delivery systems (DDS) that allow spatially and temporally controlled release of drugs are of particular interest in the field of drug delivery. These systems create opportunities for individually tailored doses of drugs to be administered as well as reduce side effects by localizing the initial drug dose to the organ of interest. We present an electroresponsive DDS in the form of a bioresorbable nanocomposite film which operates at low voltages (<-2 V). The method is based on electrochemically generating local pH changes at an electrode surface to induce dissolution of a pH-sensitive polymer, which is used as the carrier material. We previously demonstrated this proof-of-concept using a poly(methyl methacrylate-co-methacrylic acid) (co-PMMA) copolymer commercially marketed as Eudragit S100 (EGT). However, as EGT is soluble at a pH above 7, experiments were performed in isotonic saline solutions (pH 6.4). In this work, we have synthesized co-PMMA with a variety of monomer ratios to shift the solubility of the copolymer to higher pH values, and developed a polymer that can be used under physiologically relevant conditions. The generalizability of this system was demonstrated by showing controlled release of different drug molecules with varying parameters like size, hydrophobicity, and pKa. Fluorescein, a hydrophilic model compound, meloxicam, a hydrophobic anti-arthritic medication, curcumin, a small molecule with anti-cancer therapeutic potential, and insulin, a polypeptide hormone used in the treatment of hypoglycemia, could all be released on demand with minimal leakage. The drug loading achieved was 32 wt% by weight of the co-polymer.
View details for PubMedID 29781009
Visible light photoswitching of conjugated polymer nanoparticle fluorescence
2016; 52 (22): 4144–47
Conjugated polymer nanoparticles doped with a reverse photochromic dye exhibit highly quenched fluorescence that can be reversibly activated by controlling the form of the photochrome with visible light.
View details for DOI 10.1039/c6cc00001k
View details for Web of Science ID 000372175700006
View details for PubMedID 26838513
Functionalization of Conjugated Polymer Nanoparticles for Fluorescence Photomodulation
2014; 30 (48): 14658–69
The emission of conjugated polymer nanoparticles (CPNs or Pdots) is often tailored for specific uses by functionalizing CPNs with dyes that act as fluorescence resonance energy transfer (FRET) acceptors. A number of dye functionalization methods for CPNs have been developed, ranging from simple noncovalent doping to covalent attachment. We seek to develop guidelines for when noncovalent doping is acceptable and when covalent attachment is necessary to achieve the desired result. We present results of CPNs functionalized with photochromic spirooxazines by four different methods: simple doping, doping with an amphiphilic coating polymer, covalent functionalization prior to CPN formation, and covalent functionalization after CPN formation. The different CPNs are evaluated in terms of their fluorescence photomodulation properties to determine how the preparation method affects the CPN-dye photophysical interactions. Doping preparations yield the most efficient quenching of CPN emission due to shorter donor-acceptor distances in these CPNs compared to those with covalently tethered dyes. Aging studies reveal that the photochromic dyes in doped samples degrade over time to a far greater extent than those in covalently functionalized samples. These results suggest that dye-doped CPNs are appropriate for short-term experiments where highly efficient FRET is desired while covalent dye functionalization is a better choice for experiments executed over an extended time frame.
View details for DOI 10.1021/la503823v
View details for Web of Science ID 000346325700034
View details for PubMedID 25406070
- Direct synthesis of hollow polymeric nanocapsules of variable shell thickness and rigidity RSC ADVANCES 2013; 3 (29): 11525–28