Detection of small molecules or proteins of living cells provides an exceptional opportunity to study genetic variations and functions, cellular behaviors, and various diseases including cancer and microbial infections. Our aim in this review is to give an overview of selected research activities related to nucleic acid-based aptamer techniques that have been reported in the past two decades. Limitations of aptamers and possible approaches to overcome these limitations are also discussed.
The coffee-ring effect, ubiquitously present in the drying process of aqueous droplets, impedes the performance of a myriad of applications involving precipitation of particle suspensions in evaporating liquids on solid surfaces, such as liquid biopsy combinational analysis, microarray fabrication, and ink-jet printing, to name a few. We invented the methodology of laser-induced differential evaporation to remove the coffee-ring effect. Without any additives to the liquid or any morphology modifications of the solid surface the liquid rests on, we have eliminated the coffee-ring effect by engineering the liquid evaporation profile with a CO2 laser irradiating the apex of the droplets. The method of laser-induced differential evaporation transitions particle deposition patterns from coffee-ring patterns to central-peak patterns, bringing all particles (e.g. fluorescent double strand DNAs) in the droplet to a designated area of 100 μm diameter without leaving any stains outside. The technique also moves the drying process from the constant contact radius (CCR) mode to the constant contact angle (CCA) mode. Physical mechanisms of this method were experimentally studied by internal flow tracking and surface evaporation flux mapping, and theoretically investigated by development of an analytical model.
Nucleic acid detection and quantification technologies have made remarkable progress in recent years. Among existing platforms, hybridization-based assays have the advantages of being amplification free, low instrument cost, and high throughput, but are generally less sensitive compared to sequencing and PCR assays. To bridge this performance gap, we developed a quantitative physical model for the hybridization-based assay to guide the experimental design, which leads to a pico-liter droplet environment with drastically enhanced performance and detection limit several order above any current microarray platform. The pico-liter droplet hybridization platform is further coupled with the on-chip enrichment technique to yield ultrahigh sensitivity both in terms of target concentration and copy number. Our physical model, taking into account of molecular transport, electrostatic intermolecular interactions, reaction kinetics, suggests that reducing liquid height and optimizing target concentration will maximize the hybridization efficiency, and both conditions can be satisfied in a highly parallel, self-assembled pico-liter droplet microarray that produces a detection limit as low as 570 copies and 50 aM. The pico-liter droplet array device is realized with a micropatterned superhydrophobic black silicon surface that allows enrichment of nucleic acid samples by position-defined evaporation. With on-chip enrichment and oil encapsulated pico-liter droplet arrays, we have demonstrated a record high sensitivity, wide dynamic range (6 orders of magnitude), and marked reduction of hybridization time from >10 h to <5 min in a highly repeatable fashion, benefiting from the physics-driven design and nanofeatures of the device. The design principle and technology can contribute to biomedical sensing and point-of-care clinical applications such as pathogen detection and cancer diagnosis and prognosis.
The inner structure, especially the nuclear structure, of cells carries valuable information about disease and health conditions of a person. Here we demonstrate a label-free technique to enable direct observations and measurements of the size, shape and morphology of the cell nucleus. With a microfabricated lens and a commercial CMOS imager, we form a scanning light-sheet microscope to produce a dark-field optical scattering image of the cell nucleus that overlays with the bright-field image produced in a separate regime of the same CMOS sensor. We have used the device to detect nuclear features that characterize the life cycle of cells and have used the nucleus volume as a new parameter for cell classification. The device can be developed into a portable, low-cost, point-of-care device leveraging the capabilities of the CMOS imagers to be pervasive in mobile electronics.
Detection of low abundance biomolecules is challenging for biosensors that rely on surface chemical reactions. For surface reaction based biosensors, it require to take hours or even days for biomolecules of diffusivities in the order of 10(-10-11) m2/s to reach the surface of the sensors by Brownian motion. In addition, often times the repelling Coulomb interactions between the molecules and the probes further defer the binding process, leading to undesirably long detection time for applications such as point-of-care in vitro diagnosis. In this work, we designed an oil encapsulated nanodroplet array microchip utilizing evaporation for pre-concentration of the targets to greatly shorten the reaction time and enhance the detection sensitivity. The evaporation process of the droplets is facilitated by the superhydrophilic surface and resulting nanodroplets are encapsulated by oil drops to form stable reaction chamber. Using this method, desirable droplet volumes, concentrations of target molecules, and reaction conditions (salt concentrations, reaction temperature, etc.) in favour of fast and sensitive detection are obtained. A linear response over 2 orders of magnitude in target concentration was achieved at 10 fM for protein targets and 100 fM for miRNA mimic oligonucleotides.
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