**1. Introduction**

Biosensors have broad applications in drug discovery, medical diagnostics [1,2], environmental monitoring and food safety [3]. The concentration of specific analytes and their reaction kinetics can be identified in biosensors [4,5]. Biosensors using microdroplets have offered reconfigurability and flexibility and limited contamination during sample preparation and analysis [4]. In this review, we focus on the use of droplets to improve the temporal, spatial and chemical resolution of biosensing measurements by compartmentalizing samples during transport from the sampling site, to the analytical site. In doing so, it is possible to reduce Taylor dispersion.

In 1953, Taylor dispersion was described as "the combined action of molecular diffusion and the variation of velocity over the cross section" [6]. Because of this dispersive phenomena, the distance that chemical signals (variation in concentration with time and space) or analytes can be transported in the continuous phase is severely limited [6,7]. The top part of Figure 1 shows pulses of imaginary chemical A and chemical B. Initially, the pulses are separated from one another, but after a short distance the chemical pulses have merged and spread out [8]. In contrast, the bottom part of Figure 1 [8] shows that pulses of chemical A and B, when contained in droplets are not dispersed or merged after transport. The sampling of tiny volumes of aqueous body fluids and their transport to systems for precise detection or quantification is an ongoing area of activity in biomedical research [9–25].

The chemical content of a sample taken from a biological system may change, depending on when and where the sample is taken. For example, changes in neurotransmitter concentrations in the extracellular space around synapses are known to happen in milliseconds to seconds [26–29]. It is also well known that several neurotransmitters coexist in a given synaptic region, and that they can be

released at different times [28,29]. It has been shown by Bert [30] that glutamate changes occurring in 1 min can be completely dampened when samples are pooled, as opposed to discretized.

**Figure 1.** Schematic representation of using segmented flow to avoid the Taylor dispersion. (The vertical white lines represent the passage of space and time (Reprinted with permission from Reference [8]. Copyright 2011 Royal Society of Chemistry).

It is reported that temporal resolution for conventional high-performance liquid chromatography (HPLC) is minutes [31]. Such time scales make it impossible to quantify analyte concentrations that change rapidly accurately. Droplet-based approaches can be used to effectively avoid dispersion and improve temporal resolution by capturing and storing events that occur too quickly for a particular analytical method [32–37]. Microdialysis sampling, coupled with droplets and direct infusion mass spectrometry was used for acetylcholine monitoring [38] with intervals of just a few seconds. Monitoring of real-time streptavidin–biotin binding kinetics was also achieved using droplet microfluidics integrated with confocal spectroscopy [39]. Srinivasan et al. [40] reported the integration of optical absorbance measurements with droplet-based microfluidics for the detection of glucose using glucose oxidase in less than 40 s. The coupling of a digital droplet-based microfluidic device to surface plasmon resonance (SPR) imaging has also been demonstrated [41,42].

Water-in-oil droplets are not perfect containers. Water and small molecules move through the water-oil interface at a non-zero rate [43]. Surfactants at the interface may form micelles, leading to another mechanism for analytes to escape droplets. Air bubbles have been used as separators between aqueous slugs to reduce cross contamination [44], and leakage is suspected to be worse in the corners of rectangular channels [45].

Conventional sampling tools, which can undertake a continuous sampling of body fluid, cannot sample and transport rapid changes of chemical signals from the insertion point to an analytical instrument without signal distortion, because of the Taylor dispersion phenomenon. Similarly, the same signal distortion problem also occurs when delivering sequences of different drugs to the injection point. A real-time analytical chemistry lab, small enough to fit inside the brain of a mouse does not exist. However, we can achieve similar analytical aims if we can digitize the liquid environment from precise locations within an organism at precise times using water-based liquids, carried by an immiscible oil in a hydrophobic channel. In this paper, we review state-of-the-art low-volume sampling probes that use droplets to transport signals for downstream analysis. For these sampling tools, hydrophobic and hydrophilic surfaces are used to control the movement of liquids.
