Modeling Graphene-Polymer Heterostructure MEMS Membranes with the Föppl-von Kármán Equations.

Ultra-thin graphene-based membranes have shown significant promise for high-performance nano-electro-mechanical (NEMS) devices. The key challenge in the modeling of such membranes is that they often operate in deflection regimes where the assumptions or approximations of "pure bending" or "pure stretching" are not satisfied. We present a model of graphene-polymer heterostructure (GPH) NEMS membranes based on Föppl-von Kármán (FvK) equations which take into account both bending and stretching forces. The experimental GPH membrane shape obtained through atomic force microscopy topography mapping is compared to the inflation shapes predicted by FvK-based finite element method simulation, and they show excellent agreement with each other. When the GPH membranes are deflected under pressure in a capacitive pressure sensor configuration, the effectiveness of this model is further exemplified through accurately predicting the capacitance change of deflecting GPH membrane devices at varying pressures. This model serves as a powerful new tool in the design and development of graphene-based NEMS devices, being able to predict the performance of graphene NEMS devices or to aid in the design of device geometries to match required performances.

Development of an open-source thermally stabilized quartz crystal microbalance instrument for biomolecule-substrate binding assays on gold and graphene.

The interaction of biomolecules, such as proteins, with biomaterial surfaces is key to disease diagnostic and therapeutic development applications. There is a significant need for rapid, low-cost, field-serviceable instruments to monitor such interactions, where open-source tools can help to improve the accessibility to disease screening instruments especially in low- and middle-income countries. We have developed and evaluated a low-cost integrated quartz crystal microbalance (QCM) instrument for biomolecular analysis based on an open-source QCM device. The custom QCM instrument was equipped with a custom-made electronically controlled isothermal chamber with a closed-loop control routine. A thermal coefficient of 5.6 ppm/°C was obtained from a series of evaluations of the implemented control. Additionally, a custom-designed data acquisition system and a mathematical processing and analysis tool is implemented. The quartz crystal detection chips used here incorporate gold and reduced graphene oxide (rGO) coated surfaces. We demonstrate the system capability to monitor and record the biomolecular interaction between a typical protein bovine serum albumin (BSA) and these two substrates. This instrument was compared to a commercial QCM, demonstrating good correspondence between the computed mass adsorption density responses using the Sauerbrey model. For both Au and rGO surfaces, the custom QCM significantly outperforms the commercial system in limit of detection, sensitivity and linear range. The instrument presented here has the potential to serve as a ubiquitous bioelectronic tool for point-of-care disease screening and rapid therapeutics development.

A Point-of-Care Immunosensor Based on a Quartz Crystal Microbalance with Graphene Biointerface for Antibody Assay.

We present a sensitive and low-cost immunoassay, based on a customized open-source quartz crystal microbalance coupled with graphene biointerface sensors (G-QCM), to quantify antibodies in undiluted patient serum. We demonstrate its efficacy for a specific antibody against the phospholipase A2 receptor (anti-PLA2R), which is a biomarker in primary membranous nephropathy. A novel graphene-protein biointerface was constructed by adsorbing a low concentration of denatured bovine serum albumin (dBSA) on the reduced graphene oxide (rGO) sensor surface. The dBSA film prevents the denaturation of the protein receptor on the rGO surface and serves as the cross-linker for immobilization of the receptor for anti-PLA2R antibodies on the surface. The detection limit and selectivity of this G-QCM biosensor was compared with a commercial QCM system. The G-QCM immunoassay exhibited good specificity and high sensitivity toward the target, with an order of magnitude better detection limit (of 100 ng/mL) compared to the commercial system, at a fraction of the cost and with considerable time saving. The results obtained from patient sera compared favorably with those from enzyme-linked immunosorbent assay, validating the feasibility of use in clinical applications. The multifunctional dBSA-rGO platform provides a promising biofunctionalization method for universal immunoassay and biosensors. With the advantages of inexpensive, rapid, and sensitive detection, the G-QCM sensor and instrument form an effective autoimmune disease screening tool.