Photoelectrochemical Sensor Tests for Coronavirus in Minutes

Scientists in Spain have designed a photoelectrochemical aptasensor to quickly detect coronavirus using a saliva sample. The sensor device, developed by a research team at the Universidad Carlos III de Madrid (UC3M), obtains quantitative measurements of the severe acute respiratory syndrome coronavirus-2 receptor-binding domain (SARS-CoV-2 RBD).

Electrochemical aptasensors are biosensors that use an aptamer probe as the bio-recognizer element on the surface of a working electrode. Aptamers bind to their target with high affinity and specificity. As the target binds to the immobilized aptamer on the surface of the electrode, the electrode converts the aptamer-target interaction into a diagnostic electrical signal.

A sample of the patient’s saliva is dissolved in a buffer solution and placed on to the sensor’s surface. Test results are available within a few minutes.

Because aptamers are about 10× smaller than antibodies, more aptamers can be immobilized on the surface of the transducer. “The advantage over current antigen-based sensors is the greater sensitivity and specificity of the photoelectrochemical sensor measurements, which are comparable to more complex measurements, such as those from fluorescence-based sensors,” researcher Mahmoud Amouzadeh Tabrizi said. “And, they are simpler, cheaper, and faster than PCR-based sensors.”

A photoelectrochemical aptasensor for the quantitive measurement of the severe acute respiratory syndrome coronavirus-2 receptor-binding domain (Sars-CoV-2 RBD) has been introduced by a Spanish research team. The mechanism detects from human saliva samples. Courtesy of UC3M.

The researchers fabricated, characterized, and designed graphitic carbon nitride and (gC₃N₄) and cadmium sulfide (CdS) quantum dots. They added the CdS-gC₃N₄ nanocomposite to a solution containing an amine-rich polymer (chitosan). They then modified the surface of the electrode with the chitosan/CdS-gC₃N₄ solution and immobilized the aptamer probes on the electrode’s surface.

The aptamer/chitosan/CdS QDs-gC₃N₄/ITO electrode demonstrated exceptional analytical performance in terms of dynamic response range, low detection limit, selectivity, sensitivity, and stability.

“We used a surface that contains graphitic carbon nitride-cadmium sulfide quantum dots (C₃N₄-CdS) with photoactive properties. It is on this surface that a specific receptor is immobilized in such a way that, in the presence of the target molecule, it binds to the bioreceptor, thereby reducing the current generation associated with the presence of light,” Tabrizi said. “On this particular sensor, the bioreceptor used is an aptamer that is capable of interacting with the receptor-binding domain (RBD) of the SARS-CoV-2 virus.”

The researchers studied the electrochemical performance of the electrodes using cyclic voltammetry, electrochemical impedance spectroscopy, and photoelectrochemistry. They characterized the surface of the material and the receiver that was immobilized on the surface using Fourier-transform infrared spectroscopy (FTIR) and two microscopy techniques.

“The results obtained from using all of these techniques allow us to ensure that both the manufacture of the desired photosensitive nanomaterial and the immobilization of the bioreceptor have been properly carried out,” professor Pablo Acedo said.

The researchers used the photoelectrochemical aptasensor to measure the spiked SARS-CoV-2 RBD in human saliva samples at two concentrations.

The new aptasensor is able to detect virus concentrations below 0.5 nanomolars (nM), which is typical in patients who have not yet developed COVID symptoms, and at higher concentrations up to 32 nM. Because it exhibits a wide range of sensitivity to different virus concentrations, the aptasensor could be used as a tool for monitoring the progress of infection in patients.

The aptasensor is not only more sensitive than antigen-based sensors; it also detects the virus more quickly and cost-effectively than polymerase chain reaction (PCR) tests. It can be integrated with portable diagnostic systems and is easy to use.

Acedo said that the team will complement its photoelectrochemical aptasensor with the development of comprehensive biomedical instruments and diagnostics, to create a highly sensitive, highly specific, portable, and potentially low-cost diagnostic system that can be used in clinical practice.

“We are seeking a diagnosis similar to those currently available when reading blood glucose levels in patients with diabetes, for example,” he said. “We are also aiming to contact companies that may be interested in these developments.”

The research was published in two publications: Sensors and Actuators B: Chemical (www.doi.org/10.1016/j.snb.2021.130377) and Biosensors and Bioelectronics (www.doi.org/10.1016/j.bios.2021.113729).