Metal ions such as copper (II) (Cu(II)) are imperative for normal brain health and function, including as cofactors for enzymes, signaling molecules, factors for neurite growth, modulators of vesicular trafficking, and regulators of signaling receptors . Glutamate-induced activation of N-methyl-D-aspartate (NMDA)-receptors is hypothesized to trigger the translocation of copper-bound transporters to the synapse (on millisecond timescale) . This results in free Cu(II) being released into the synaptic cleft, the nanometre-sized gap between two communicating neurons. In one hypothesis, this Cu(II) release prevents the over-excitability of the NMDA receptors from excessive glutamate , a phenomenon that is toxic to the neurons and causes cell death. High glutamate release, NMDA activation, and resulting excitotoxicity are hypothesized to be one of the significant causes of neurodegeneration in disorders such as Alzheimer's disease (AD) 2 (reviewed in Parameshwaran et al. ). One of the hallmarks of AD is the aggregation of amyloid-b into senile plaques. Studies have indicated that Cu(II) is among the primary cations found inside senile plaques . Interestingly, some studies suggest that Cu(II) binding to amyloid-b makes it more prone to aggregation and increases the production of reactive oxygen species , thereby leading to cognitive decline and neurodegeneration. It has even been suggested that amyloid-b is increased to combat toxic levels of metal ions such as Cu(II) . There is thus still an ongoing debate on whether Cu(II) is neuroprotective (preventing excitotoxicity) or deleterious (stabilizing amyloid aggregates) in the context of neurodegeneration. The role of Cu(II) in AD is generally controversial, as some studies observed a copper deficiency in AD patients while others show a copper overload (also reviewed in Ejaz et al. and Bagheri et al. ). Moreover, studies have revealed different Cu content in different regions of the brain . This is important because the hippocampus and cortex, for example, are more susceptible to plaque formation . Copper quantitation in biological samples is commonly accomplished by separations and spectroscopic techniques with high sensitivity. Separation methods like high-performance liquid chromatography (HPLC) and inductively coupled plasma mass spectrometry are championed for their multiplexing capabilities and are widely utilized in analysing clinical samples for copper . However, these methods are restricted to measuring total copper concentration (free Cu and those bound to proteins) and require multiple sample preparation steps (including sampling, extraction, digestion, and dilution) and longer analysis time. On the other hand, while colorimetric assays are cost-effective and have a more rapid readout than HPLC , they are limited by analyte-chromogen interaction kinetics . Fluorescent probes have faster response times and have been deployed in cells but rely on complicated fluorophore design and synthesis. Additionally, fluorescence assays are limited in their time resolution as they require incubation for 30 min-1h for the fluorophore to localize appropriately. Probe-free methods like surface-enhanced Raman spectroscopy has also been applied to quantify Cu(II) in aqueous medium , but its relatively longer data acquisition time prevents its applicability to study shorter time-scale phenomena. Many of these studies above quantify total Cu content, i.e., bound and free Cu together, rather than free Cu(II) alone. The majority of Cu(II) is usually in the bound form and coordinated to biomolecules (i.e., protein) . While total Cu(II) may not change much or as rapidly, free Cu(II) can in response to