Microcompartments, such as aerosols, cells, and geological pores, are ubiquitous in nature and often exhibit unique chemical behaviors compared to the bulk phase. Reactions occurring in these confined spaces frequently proceed at vastly different rates than in bulk environments, which has important implications for industrial synthesis, atmospheric modeling, and cellular processes. Despite this, the mechanisms behind this distinct behavior remain poorly understood. The goal of this work is to investigate the mechanisms behind the modification of reaction rates in microdroplets.In Chapter 2 of this work, reaction kinetics are measured in individual aqueous droplets to minimize external variables and explore the mechanisms driving altered reaction rates. The experiments are conducted using a quadrupole electrodynamic trap (QET), which enables observation of single droplets in a controlled environment. Using fluorescence measurements, the reaction between dopamine and resorcinol is investigated in droplets and compared to bulk cuvette measurements across a range of conditions. The reaction is found to be significantly accelerated in droplets, and kinetic modeling reveals that this acceleration is due to rapid oxygen diffusion into aerosol droplets and increased reactant concentrations at the droplet interface. In order to explore other mechanisms behind reaction acceleration in droplets, nanodiamond (ND) sensors are deployed in droplets.Chapters 3 and 4 explore the use of NDs with nitrogen-vacancy (NV) defects for sensing in microcompartments. In particular, the potential of NDs as sensors for paramagnetic species in single droplets is explored to increase the range of possible measurements in the QET. In Chapter 3, the influence of environmental factors, such as pH, on the paramagnetic sensing properties of these sensors is examined using gadolinium (Gd3+), a highly paramagnetic ion, as a test case. It is determined that Gd3+ must bind to the ND surface to be effectively sensed. A comprehensive model is developed to predict trends in paramagnetic species sensing accounting for pH, competitive binding, nanodiamond size, and depletion effects. This model highlights the sensitivity of these NDs to any species in solution that can impact Gd3+ binding to the diamond surface. While quantitative determination of paramagnetic species concentrations in complex matrices is possible, it requires extensive calibration.In Chapter 4, the transmetalation reaction of gadolinium (III) diethylenetriaminepentaacetic acid (Gd-DTPA) with zinc is studied in both droplets and bulk solution in order to evaluate the effectiveness of commercially available ND sensors for measuring reaction kinetics in microcompartments. In the bulk, reaction kinetics are also benchmarked against nuclear magnetic resonance (NMR) measurements using established procedures. While the sensors successfully capture trends in reaction rates in droplets and in the bulk environment, concentrations of Gd3+ are underestimated by the ND sensors. A significant deceleration of the reaction is observed in droplets compared to the bulk. Correcting for differences in pH and acetic acid concentration between the two environments cannot account for this deceleration which is attributed to a property of the droplet interface. In Chapter 5, the future of these NDs for sensing in microcompartments is discussed. In order to improve their ability to quantify paramagnetic species concentrations, targeted surface functionalization is likely needed
