pH Sensing Properties of Flexible, Bias-Free Graphene Microelectrodes in Complex Fluids: From Phosphate Buffer Solution to Human Serum

Multifunctional carbon dots for pH sensing, intracellular imaging, amino acid classification, and test strip development

Intracellular pH is a key indicator of cellular stability and plays a critical role in understanding biological processes and diagnosing various diseases. In this study, yellow-emitting carbon dots (C-dots) were synthesized via hydrothermal treatment of o-phenylenediamine and p-aminobenzoic acid. These C-dots exhibited pH-sensitive fluorescence, with intensity increasing as the pH ranged from 2.0 to 10.0, displaying a pKa value of 5.14 and a linear response between pH 4.0 and 6.0. The observed pH sensitivity is mainly due to the abundant carboxyl groups on the C-dots' surface, which undergo protonation, deprotonation, and electrostatic transfer in response to pH changes. Additionally, these C-dots effectively detected both acidic and alkaline amino acids in aqueous solutions and on test strips. Notably, they were successfully used for intracellular imaging of amino acids, offering a significant advantage over toxic small-molecule reagents that can alter pH levels in living cells. These pH-sensitive C-dots have the potential to enhance applications in both analytical detection and biological imaging, expanding their use in variety of fields.

Flow-sensory contact electrification of graphene

All-electronic interrogation of biofluid flow velocity by electrical nanosensors incorporated in ultra-low-power or self-sustained systems offers the promise of enabling multifarious emerging research and applications. However, existing nano-based electrical flow sensing technologies remain lacking in precision and stability and are typically only applicable to simple aqueous solutions or liquid/gas dual-phase mixtures, making them unsuitable for monitoring low-flow (~micrometer/second) yet important characteristics of continuous biofluids (such as hemorheological behaviors in microcirculation). Here, we show that monolayer-graphene single microelectrodes harvesting charge from continuous aqueous flow provide an effective flow sensing strategy that delivers key performance metrics orders of magnitude higher than other electrical approaches. In particular, over six-months stability and sub-micrometer/second resolution in real-time quantification of whole-blood flows with multiscale amplitude-temporal characteristics are obtained in a microfluidic chip.

All-Electronic Quantification of Neuropeptide-Receptor Interaction Using a Bias-Free Functionalized Graphene Microelectrode

Opioid neuropeptides play a significant role in pain perception, appetite regulation, sleep, memory, and learning. Advances in understanding of opioid peptide physiology are held back by the lack of methodologies for real-time quantification of affinities and kinetics of the opioid neuropeptide-receptor interaction at levels typical of endogenous secretion (<50 pM) in biosolutions with physiological ionic strength. To address this challenge, we developed all-electronic opioid-neuropeptide biosensors based on graphene microelectrodes functionalized with a computationally redesigned water-soluble μ-opioid receptor. We used the functionalized microelectrode in a bias-free charge measurement configuration to measure the binding kinetics and equilibrium binding properties of the engineered receptor with [d-Ala², N-MePhe⁴, Gly-ol]-enkephalin and β-endorphin at picomolar levels in real time.