Significant progress has been made on the development of electrolyte-gated graphene field effect transistor (EGGFET) biosensors over the last decade, yet they are still in the stage of proof-of-concept. In this work, we studied the electrolyte matrix effects, including its composition, pH and ionic strength, and demonstrate that variations in electrolyte matrices have a significant impact on the Fermi level of the graphene channel and the sensitivity of the EGGFET biosensors. This is attributed to the polarization-induced interaction between the electrolyte and the graphene at the interface which can lead to considerable modulation of the Fermi level of the graphene channel. As a result, the response of the EGGFET biosensors is susceptible to the matrix effect which might lead to high uncertainty or even false results. Then, an EGGFET immunoassay is presented which aims to allow good regulation of the matrix effect. The multichannel design allows in-situ calibration with negative control, as well as statistical validation of the measurement results. Its performance is demonstrated by the detection of human immunoglobulin G (IgG) from serum. The detection range is estimated to be around 2–50 nM with a coefficient of variation (CV) of less than 20% and the recovery rate for IgG detection is around 85–95%. Compared with traditional immunoassay techniques, the EGGFET immunoassay is label-free and ready to be integrated with microfluidics sensor platforms, suggesting its great prospect for point-of-care applications.

Poly(methyl methacrylate) (PMMA) residue has long been a critical challenge for practical applications of the transferred chemical vapor deposited (CVD) graphene. Thermal annealing is empirically used for the removal of the PMMA residue; however experiments imply that there are still small amounts of residues left after thermal annealing which are hard to remove with conventional methods. In this paper, the thermal degradation of the PMMA residue upon annealing was studied by Raman spectroscopy. The study reveals that post-annealing residues are generated by the elimination of methoxycarbonyl side chains in PMMA and are believed to be absorbed on graphene via the π – π interaction between the conjugated unsaturated carbon segments and graphene. The post-annealing residues are difficult to remove by further annealing in a non-oxidative atmosphere due to their thermal and chemical stability. An electrolytic cleaning method was shown to be effective in removing these post-annealing residues while preserving the underlying graphene lattice based on Raman spectroscopy and atomic force microscopy studies. Additionally, a solution-gated field effect transistor was used to study the transport properties of the transferred CVD graphene before thermal annealing, after thermal annealing, and after electrolytic cleaning, respectively. The results show that the carrier mobility was significantly improved, and that the p-doping was reduced by removing PMMA residues and post-annealing residues. These studies provide a more in-depth understanding on the thermal annealing process for the removal of the PMMA residues from transferred CVD graphene and a new approach to remove the post-annealing residues, resulting in a residue-free graphene.

We report a unique constant phase element (CPE) behavior ( $\frac{1}{Z}=Q_0{\left(j\omega \right)}^{\alpha }$ ) of the electrolyte–graphene interface with both $Q_0$ and $\alpha$ showing dependence on the gate voltage. The frequency response of the electrolyte–graphene interface was studied using electrochemical impedance spectroscopy (EIS). The result suggests that (1) the electrolyte–graphene interface should be characterized as a CPE ( $\alpha$ < 1), rather than an ideal capacitor; and (2) both $Q_0$ and $\alpha$ show ambipolar dependence on the applied voltage. We speculate that the CPE behavior of the electrolyte–graphene interface arises from the charged impurities on the substrate and the defects in the graphene lattice, which could introduce inhomogeneity of local density of states (DOS). The low density of states of graphene makes $\alpha$ sensitive to these local DOS near the Dirac point, and thus showing dependence on the gate voltage. Measurement of the electrolyte–graphene interface capacitance based on multi-frequency capacitance-voltage (CV) profiling was demonstrated, and the extraction of the carrier mobility was performed. The study could lead to a more accurate understanding of the capacitive behavior of the electrolyte–graphene interface, which is instructive for the design and analysis of devices involving the electrolyte–graphene interface for nanoelectronics and bioelectronics applications.

Microfluidic technologies enable in vitro studies to closely simulate in vivo microvessel environment with complexity. Such method overcomes certain constrains of the statically cultured endothelial monolayers and enables the cells grow under physiological range of shear flow with geometry similar to microvessels in vivo. However, there are still existing knowledge gaps and lack of convincing evidence to demonstrate and quantify key biological features of the microfluidic microvessels. In this paper, using advanced micromanufacturing and microfluidic technologies, we presented an engineered microvessel model that mimicked the dimensions and network structures of in vivo microvessels with a long-term and continuous perfusion capability, as well as high-resolution and real-time imaging capability. Through direct comparisons with studies conducted in intact microvessels, our results demonstrated that the cultured microvessels formed under perfused conditions recapitulated certain key features of the microvessels in vivo. In particular, primary human umbilical vein endothelial cells were successfully cultured the entire inner surfaces of the microchannel network with well-developed junctions indicated by VE-cadherin staining. The morphological and proliferative responses of endothelial cells to shear stresses were quantified under different flow conditions which was simulated with three-dimensional shear dependent numerical flow model. Furthermore, we successfully measured agonist-induced changes in intracellular Ca²⁺ concentration and nitric oxide production at individual endothelial cell levels using fluorescence imaging. The results were comparable to those derived from individually perfused intact venules. With in vivo validation of its functionalities, our microfluidic model demonstrates a great potential for biological applications and bridges the gaps between in vitro and in vivo microvascular research.

Sample enrichment or molecules concentration is considered an essential step in sample processing of miniaturized devices aimed at biosensing and bioanalysis. Among all the means involved to achieve this aim, dielectrophoresis (DEP) is increasingly employed in molecules manipulation and concentration because it is non-destructive and high efficiency. This paper presents a methodology to achieve protein concentration utilizing the combination effects of electrokinetics and low frequency insulating dielectrophoresis (iDEP) generated within a microfluidic device, in which a submicron constricted channel was fabricated using DNA molecular combing and replica molding. This fabrication technique avoids using e-beam lithography or other complicated nanochannel fabrication methods, and provides an easy and low cost approach with the flexibility of controlling channel dimensions to create highly constricted channels embedded in a microfluidic device. With theoretical analysis and experiments, we demonstrated that fluorescein isothiocyanate conjugated bovine serum albumin (FITC-BSA) protein molecules can be significantly concentrated to form an arc-shaped band near the constricted channel under the effects of a negative dielectrophoretic force and DC electrokinetic forces within a short period of time. It was also observed that the amplitudes of the applied DC and AC electric fields, the AC frequencies as well as the suspending medium conductivities had strong effects on the concentration responses of the FITC-BSA molecules, including the concentrated area and position, intensities of the focused molecules, and concentration speed. Our method provides a simple and flexible approach for quickly concentrating protein molecules by controlling the applied electric field parameters. The iDEP device reported in this paper can be used as a stand-alone sensor or worked as a pre-concentration module integrated with biosensors for protein biomarker detection. Furthermore, low frequency dielectrophoresis provides practical uses for integrating the concentration module with a portable biosensing system.