Electrochemical Biosensors for the Detection of Viruses: Must-Have Products or Just Science for Publication?

Abstract

The SARS-CoV-2 pandemic has brought significant light to the urgent need for rapid, precise, and low-cost diagnosis tools. The scientific community has responded as quickly, overflowing the literature with papers describing interesting biosensors for aiding in the diagnosis of COVID-19.1,2 However, almost none of them, mainly the electrochemical ones have reached the market or never will, with only a few traditional formats used in the daily combat of the virus, including ELISA (enzyme-linked immunosorbent assay), lateral flow assays, and, mainly, PCR (polymerase chain reaction). Although PCR-based methods are currently the gold standard for detecting viruses worldwide, these still present various drawbacks. Usually, the commercial detection of viruses (such as SARS-CoV-2) uses the combination of standard PCR (or RT-PCR) and gel electrophoresis due to its sensitivity, reliability, and low price (if compared to other PCR-based methods such as real-time PCR). This approach relies, mainly, on the use of a standard thermal cycler and an electrophoresis tank by a specialized worker. While electrophoresis tanks can be quite affordable, with some of them costing a few hundred dollars,3 even simple thermal cyclers cost around 5,000 USD4 – significantly enhancing the investment required for testing. Furthermore, the complete analysis of a sample is slow and can take up to six hours to complete, which prevents an effective sanitary barrier at borders and crowded events, for example. The samples need to be transported to the lab, as no reliable portable PCR and gel electrophoresis equipment are available. The results commonly take from two to five days to be generated - an extremely long delay when considering that these can seriously influence the health of a patient and the spread of the virus. Last, standard PCR does not provide quantitative information – which is vital in some cases to aid in diagnosing the severity of an infection. Techniques derived from PCR (such as qPCR, for example), on the other hand, can provide quantitative and more rapid results, but are also more expensive and still require sample transportation. Equipment for performing qPCR ranges from 15,000 USD to 90,000 USD4 and the use of specific reaction kits containing fluorescent markers also corresponds to a significant increase in analysis costs. Other commercially available methods for the detection of viruses, ELISA and lateral flow assays, also present significant drawbacks. While ELISA is time demanding (6 h) and requires specialized professionals and equipment to be adequately performed, some lateral flow assays present results with low precision,5,6 being useful for massive triages in the case of COVID-19, for example. Although presenting such limitations, PCR-based techniques are still the gold standard for the detection of viruses. This is probably due to its sensitive and well-established features, being widespread along with many medical and research centers around the globe. Furthermore, the development of PCR-based diagnosis kits in urgent scenarios, such as the one imposed by SARS-CoV-2, is straightforward and allows rapid responses from health organizations and governments. The technique can also provide low limits of detection (LOD), with a gold standard RT-PCR assay for COVID-19 presenting a LOD of ~100 copies of viral RNA per mL of transport media, for example. It is important to mention, however, that the LOD of currently approved assays for COVID-19 varies over 10,000-fold, which will generate immense false-negative rates.7 Biosensors present interesting properties to overcome some of the drawbacks presented by PCR. Although thousands of papers have been published in the last years based on the detection of several diseases, almost all the material published has focused on the formation of human resources and not on the market (Table I). There are few discussions in the electrochemical meetings and a tremendous demand to produce new selling and profitable devices for the environment, food, medical, and forensic analyses. In this context, portable potentiostats are commonly available on the market at prices that range from a few thousand dollars (2,000 – 3,000 USD) for full desktop equipment8 to a few hundred dollars for equipment devoted to a single analysis. There is also significant research interest in the development of portable, miniaturized, and low-cost potentiostat, as highlighted by some articles published in recent years.9–11 Colorimetric biosensors, in turn, can rely on responses readable with the naked eye or using widespread smartphones. The use of smartphones can also contribute to compiling results and acquiring additional information such as patient location and data. Therefore, if compared to PCR-based techniques, instrumentation costs are decreased while its portability allows point-of-care analysis, significantly increasing the accessibility to tests in remote areas. Analysis time is also greatly diminished as results can be obtained in only a few minutes. Both of these features are of extreme importance when considering healthcare applications that commonly require quick or real-time responses. Furthermore, immunosensors do not require previous sample preparation even when using complex biological fluids, decreasing analysis costs and making it even more rapid. Last, biosensors can be easy to use, usually requiring lower previous preparation from the operator if compared to traditional techniques (Figure 1). Biosensors can also be readily developed in urgent scenarios, as proven with COVID-19. Numerous examples of electrochemical, colorimetric, and mass-sensitive devices for aiding in the diagnosis of the disease were described in the literature only a few months after the start of the pandemic event.1,12,13 Devices are commonly validated in biological samples, providing precise results in a rapid, cheap, and simple manner. So, a relevant question is, why are most of these devices still out of the consumers' reach?