Dopamine is an organic, electrochemical neurotransmitter that, when imbalanced, upregulated, or downregulated, encourages the onset and progression of various diseases including schizophrenia, Parkinson’s disease, and several cancers such as neuroblastoma and pheochromocytoma, among other disorders. Conventional dopamine detection methods are usually time-consuming and expensive, but new research reported recently may offer an inexpensive, faster, enzyme-free alternative by recruiting nanosensor technology. The details are published in Nano Letters.

“Unlike previous studies, we use a nanoceria-redox method that is very selective for dopamine and can also regenerate material with its multi-valence state,” says Sudipta Seal, professor of materials science and engineering at the University of Central Florida and corresponding author of the study. This is the first known study to demonstrate the selective detection of a biological fluid employing a dopamine plasmonic biosensor capable of analyzing complex biological substances without having to prepare a sample for an integrative microfluidic device. 

Conventional methods used to quantify dopamine levels entail two-step processes in which the analyte is first quantified using either enzyme-linked immunosorbent assay (ELISA) or high-performance liquid chromatography followed by detection of the analyte using techniques such as fluorometry or mass spectroscopy. However, these procedures are expensive, tedious, and time-consuming. In addition, dopamine’s low mass makes the neurotransmitter difficult to detect and presents yet another hurdle. Developing tests with high specificity and sensitivity is critical to detecting dopaminergic, or dopamine-mediated, disorders and, subsequently, diagnosing these conditions as early as possible. 

Using biosensors to detect dopamine is not a novel concept. Previous studies have found that electrochemical sensors are cheaper and detect the analyte faster than conventional methods; however, device activation due to electropolymerization-induced biofouling (a phenomenon in which a polymer biofilm formed by microorganisms, algae, or other living matter develops on the biosensor) and decreased selectivity from the oxidation of dopamine metabolites and other chemicals hinder test results.

The researchers in the present study developed an integrated, enzyme-free dopamine biosensor chip derived from a nanoplasmonic substrate and a passive plasma separator microfluid chip. For dopamine-binding site selectivity, the researchers used redox-active oxygen-deficient cerium nanoparticles (CNPs) and a passive plasma separator microfluidic chip. Unlike ELISA, CNPs are shelf-stable and do not require the intricate handling, storage, and preparation precautions necessary to preserve the functionality of the medium that the antibodies and cellular membrane receptors used in ELISA routinely demand.

The researchers employed a complementary analyte surfactant called an active nanoplasmonic substrate (NPS) fabricated from a gold hole-disk array conjoined with an asymmetric photonic cavity to target antigen function selectivity. The CNP surface promotes cerium oxidation states of Ce³⁺ and Ce⁴⁺, creating oxygen vacancies within the CNP crystal lattice in the process. This feature encourages redox reactions with compounds that exhibit electroactivity such as dopamine and other neurotransmitters such as norepinephrine and serotonin.

The Ce³⁺/Ce⁴⁺ ratio on the CNP surface modulates catalytic activity, allowing researchers to use the ratio to measure the CNP redox activity. The ratio regulates the degree to which the surface reacts with compounds that exhibit electrochemical activity;  dopamine CNP1 denotes a Ce³⁺/Ce⁴⁺ ratio >1 with more Ce⁴⁺ on the surface while CNP2 indicates a Ce³⁺/Ce⁴⁺ ratio <1 with more Ce³⁺ on the surface.

Dopamine has two absorption peaks, each corresponding to a different oxidation state. Unoxidized dopamine has an absorption peak at 281 nm, which falls within the spectrum of visible UV light. Oxidized dopamine shows a second peak that begins at 390 nm while the 281 nm peak begins to vanish almost at the same time. Ce⁴⁺ concentrations increase upon oxidation.

The sensitive NPS also facilitates imprinting and reproducibility with imprinting—features attributed to the robust and reliable characteristics of the NPS structure. The CNP-chip device requires no additional sample preparation or purification processes, as the device is able to extract blood plasma samples directly from the inlet bloodstream by coupling the biosensor to a microfluidic system.

Both dopamine and CNP formulations initially appear clear. CNP1 dopamine immediately changes from clear to dark brown once mixed while CNP2+ dopamine exhibits minimal color change. Researchers believe the color change indicates the transfer of charges that result in nanoparticle coating while decreasing the concentration of free unoxidized dopamine.

Using simulated body fluid, the device detects dopamine at 100 fM. Detection levels directly from blood were as sensitive as 1 nM without needing to prepare a sample. Compared to the common interfering species, the biosensor demonstrated a selectivity at least 5 times greater.

While the results show promise, further studies are needed to examine selectivity and to overcome sample corruption challenges such as biofouling.

“Methods for fast and non-invasive detection of dopamine at the concentration range near the blood baseline have clinical values. This paper presents an idea to fulfill this task,” says SuPing Lyu, Senior Principal Scientist and Technical Fellow at Medtronic Inc., who was not involved in this study. “It is worth to report it even though the work is only a proof of concept and has a lot of obvious challenges.”

Seal believes detecting dopamine concentrations outside a cell with a known interfering agent is a good starting place.

Read the abstract in Nano Letters.