Detection of aldehydes from degradation of lipid nanoparticle formulations using a hierarchically-organized nanopore electrochemical biosensor

Degradation of ionizable lipids in mRNA-based vaccines was recently found to deactivate the payload, demanding rigorous monitoring of impurities in lipid nanoparticle (LNP) formulations. However, parallel screening for lipid degradation in customized delivery systems for next-generation therapeutics maintains a challenging and unsolved problem. Here, we describe a nanopore electrochemical sensor to detect ppb-levels of aldehydes arising from lipid degradation in LNP formulations that can be deployed in massively parallel fashion. Specifically, we combine nanopore electrodes with a block copolymer (BCP) membrane capable of hydrophobic gating of analyte transport between the bulk solution and the nanopore volume. By incorporating aldehyde dehydrogenase (ALDH), enzymatic oxidation of aldehydes generates NADH to enable ultrasensitive voltammetric detection with limits-of-detection (LOD) down to 1.2 ppb. Sensor utility was demonstrated by detecting degradation of N-oxidized SM-102, the ionizable lipid in Moderna's SpikeVax™ vaccine, in mRNA-1273 LNP formulation. This work should be of significant use in the pharmaceutical industry, paving the way for automated on-line quality assessments of next-generation therapeutics.

Stochastic Sensing of Dynamic Interactions and Chemical Reactions with Nanopores/Nanochannels

Nanopore sensing technology is an emerging analysis method with the advantages of simple operation, high sensitivity, fast output and being label free, and it is widely used in protein analysis, gene sequencing, biomarker detection, and other fields. The confined space of the nanopore provides a place for dynamic interactions and chemical reactions between substances. The use of nanopore sensing technology to track these processes in real time is helpful to understand the interaction/reaction mechanism at the single-molecule level. According to nanopore materials, we summarize the development of biological nanopores and solid-state nanopores/nanochannels in the stochastic sensing of dynamic interactions and chemical reactions. The goal of this paper is to stimulate the interest of researchers and promote the development of this field.

Single enzyme electroanalysis

Traditional studies of enzymatic activity rely on the combined kinetics of millions of enzyme molecules to produce a product, an experimental approach that may wash out heterogeneities that exist between individual enzymes. Evaluating these properties on an enzyme-by-enzyme basis represents an unambiguous means of elucidating heterogeneities; however, the quantification of enzymatic activity at the single-enzyme level is fundamentally limited by the maximum catalytic rate, k_cat, inherent to a given enzyme. For electrochemical methods measuring current, single enzymes must turn over greater than 10⁷ molecules per second to produce a measurable signal on the order of 10^-12 A. Enzymes with this capability are extremely rare in nature, with typical k_cat values for biologically relevant enzymes falling between 1 and 10 000 s^-1. Thus, clever amplification strategies are necessary to electrochemically detect the vast majority of enzymes. This review details the progress toward the electroanalytical detection and evaluation of single enzyme kinetics largely focused on the nanoimpact method, a chronoamperometric detection strategy that monitors the change in the current-time profile associated with stochastic collisions of freely diffusing entities (e.g., enzymes) onto a microelectrode or nanoelectrode surface. We discuss the experimental setups and methods developed in the last decade toward the quantification of single molecule enzymatic rates. Special emphasis is given to the limitations of measurement science in the observation of single enzyme activity and feasible methods of signal amplification with reasonable bandwidth.

High-performance bioanalysis based on ion concentration polarization of micro-/nanofluidic devices

Micro-/nanofluidics has received considerable attention over the past two decades, which allows efficient biomolecule trapping and preconcentration due to ion concentration polarization (ICP) within nanostructures. The rich scientific content related to ICP has been widely exploited in different applications including protein concentration, biomolecules sensing and detection, cell analysis, and water purification. Compared to pure microfluidic devices, micro-/nanofluidic devices show a highly efficient sample enrichment capacity and nonlinear electrokinetic flow feature. These two unique characterizations make the micro-/nanofluidic systems promising in high-performance bioanalysis. This review provides a comprehensive description of the ICP phenomenon and its applications in bioanalysis. Perspectives are also provided for future developments and directions of this research field.

Biomolecular Crowding Arising from Small Molecules, Molecular Constraints, Surface Packing, and Nano-Confinement

The effect of macromolecular crowding on the structure, dynamics, and reactivity of biomolecules is well established and the relevant research has been extensively reviewed. Herein, we focus our discussion on crowding effects arising from small cosolvent molecules and densely packed surface conditions. In addition, we highlight recent efforts that capitalize on the excluded volume effect for various tailored biochemical and biophysical applications. Specifically, we discuss how a targeted increase in local mass density can be exploited to gain insight into the folding dynamics of the protein of interest and how confinement via reverse micelles can be used to study a range of biophysical questions, from protein hydration dynamics to amyloid formation.

Sensitive assay of protease activity on a micro/nanofluidics preconcentrator fused with the fluorescence resonance energy transfer detection technique

A fast and sensitive assay of protease activity on a micro/nanofluidics preconcentrator combining with fluorescence resonance energy transfer (FRET) detection technique has been developed in a homogeneous real-time format. First, the functionalized nanoprobes are formed by loading dye labeled protein onto gold nanoparticles (AuNPs), in which, the photoluminescence of donor dye was strongly quenched by AuNPs due to FRET mechanisms. For protease activity assay, the nanoprobes are enriched by a micro/nanofluidics preconcentrator. When the target protease is transported to the enriched nanoprobes, cleavage of protein occurs as a consequence of molecular recognition of enzyme to substrate. The release of cleavage fragments from AuNPs nanoprobes leads to the enhancement of fluorescence and enables the protease activity assay on the micro/nanofluidics chip. As a demonstration, digestion of fluorescein isothiocyanate labeled dog serum albumin (FITC-DSA) by trypsin was used as a model reaction. Because of the highly efficient preconcentration and space confinement effect, significantly increased protein cleavage rate and protease assay sensitivity can be achieved with enhanced enzyme activity. The present micro/nanofluidics platform fused with the FRET detection technique is promising for fast and sensitive bioanalysis such as immunoassay, DNA hybridization, drug discovery, and clinical diagnosis.

Insights into the "free state" enzyme reaction kinetics in nanoconfinement

The investigation of enzyme reaction kinetics in nanoconfined spaces mimicking the conditions in living systems is of great significance. Here, a nanofluidics chip integrated with an electrochemical detector has been designed for studying "free state" enzyme reaction kinetics in nanoconfinement. The nanofluidics chip is fabricated using the UV-ablation technique developed in our group. The enzyme and substrate solutions are simultaneously supplied from two single streams into a nanochannel through a Y-shaped junction. The laminar flow forms in the front of the nanochannel, then the two liquids fully mix at their downstream where a homogeneous enzyme reaction occurs. The "free state" enzyme reaction kinetics in nanoconfinement can thus be investigated in this laminar flow based nanofluidics device. For demonstration, glucose oxidase (GOx) is chosen as the model enzyme, which catalyzes the oxidation of beta-d-glucose. The reaction product hydrogen peroxide (H2O2) can be electrochemically detected by a microelectrode aligning to the end of nanochannel. The steady-state electrochemical current responding to various glucose concentrations is used to evaluate the activity of the "free state" GOx under nanoconfinement conditions. The effect of liquid flow rate, enzyme concentration, and nanoconfinement on reaction kinetics has been studied in detail. Results show that the "free state" GOx activity increases significantly compared to the immobilized enzyme and bath system, and the GOx reaction rate in the nanochannel is two-fold faster than that in bulk solution, demonstrating the importance of "free state" and spatial confinement for the enzyme reaction kinetics. The present approach provides an effective method for exploiting the "free state" enzyme activity in nanospatial confinement.

Reversible plasmonic probe sensitive for pH in micro/nanospaces based on i-motif-modulated morpholino-gold nanoparticle assembly

Exploring local pH in micro/nanoscale is fundamentally important for understanding microprocesses including the corrosion of metal and the metabolism of cell. Regular fluorescence pH probes and potentiometric electrodes show either low signal intensity or lack of spatial resolution when being applied in a micro/nanoenvironment. Here, we developed a nanoscale reversible pH probe based on the plasmonic coupling effect of i-motif modulated gold nanoparticle (AuNP) assembly. The pH probe shows a reversible and highly sensitive response to pH variation between 4.5 and 7.5. Introduction of morpholino oligomers (MO), a neutral analog of DNA, into the assembly endows the pH probe with high stability even under low salt concentration. The intense optical signal of a AuNP enables local pH to be read out not only in the micro/nanofluidic channel but also on a single i-motif-MO-AuNP assembly. Recording of the strong plasmonic resonance scattering spectrum of AuNP provides a promising method for extracting chemical information in nanospace of biological systems.

Rapid protein concentration, efficient fluorescence labeling and purification on a micro/nanofluidics chip

Fluorescence analysis has proved to be a powerful detection technique for achieving single molecule analysis. However, it usually requires the labeling of targets with bright fluorescent tags since most chemicals and biomolecules lack fluorescence. Conventional fluorescence labeling methods require a considerable quantity of biomolecule samples, long reaction times and extensive chromatographic purification procedures. Herein, a micro/nanofluidics device integrating a nanochannel in a microfluidics chip has been designed and fabricated, which achieves rapid protein concentration, fluorescence labeling, and efficient purification of product in a miniaturized and continuous manner. As a demonstration, labeling of the proteins bovine serum albumin (BSA) and IgG with fluorescein isothiocyanate (FITC) is presented. Compared to conventional methods, the present micro/nanofluidics device performs about 10(4)-10(6) times faster BSA labeling with 1.6 times higher yields due to the efficient nanoconfinement effect, improved mass, and heat transfer in the chip device. The results demonstrate that the present micro/nanofluidics device promises rapid and facile fluorescence labeling of small amount of reagents such as proteins, nucleic acids and other biomolecules with high efficiency.