Assay Development Fundamentals: Sensitivity & Selectivity

Author: Luca Vita

Electrochemical biosensors have a whole host of advantageous characteristics that currently put them at the forefront of biosensor technologies. They are able to produce rapid responses and low limits of detection even when sample volumes are low and undiluted. Additionally, the ease of miniaturisation of the biosensors makes diagnosis possible at the point of care whilst maintaining low costs (1). These attractive characteristics are the reason electrochemical biosensors dominated 70.9% of the biosensor market in 2020 (2). However, two aspects that are still looking to be further developed in electrochemical biosensors are selectivity and specificity.

Often, the two terms are used interchangeably however, there is a distinct difference when referring to them in the terms of analytical methods.

Selectivity

Selectivity in biosensors is the ability to accurately measure a target analyte in the presence of interferences in the sample matrix. This allows for analysis of complex mixtures without prior separation. Importantly, for an analytical method to be deemed selective it must be able to simultaneously determine several components individually from one another in a sample.

“Selectivity of an analytical method characterises the extent to which n given analytes can be measured simultaneously by n sensors (detecting channels) without interferences by other components and, therefore, can be detected or determined independently and undisturbedly.” (3)

Specificity

Alternatively, specificity defines the identification of a single analyte in a matrix of components, where the identification of the other components is not important.

“Specificity refers to single component analysis and means that the one individual component in a real sample can be undisturbedly measured by a specific reagent, a particular sensor or a comparable measuring system.” (3)

Applications of Selectivity and Specificity

The majority of selectivity or specificity in an electrochemical assay depends on the recognition factor. This is the antibodies, enzymes, DNA or other biological tissue that interact with the target analyte to produce an electrical signal. In specific sensors, this is mainly through antibodies, enzymes or more recently aptamers (oligonucleotide or peptide molecules that bind to a specific target molecule). They are able to distinguish a single target even in complex medium like blood serum (4). The most common commercial example of a specific sensing is the lateral flow immunochromatographic assay used in home pregnancy test. This ability to determine a single analyte in a complex medium is a critical component of electrochemical bioassays that allow them to be implemented at the point of care, without the need for preparation of samples.

Specific sensing is not always possible due to close similarities between analytes or just lack of modern technologies. Selective sensing is the next best technology that can be applied. Furthermore, specific sensing may not always be the most appropriate option. Diagnosis of some diseases requires monitoring of multiple biomarkers and therefore require an array of antibodies in a sensor (5).

A key example that differentiates selective sensing from specific sensing is the implementation of lectins to sense carbohydrates. Although lectins do have high specificity for carbohydrates in general, individual lectins are not specific to glycosylated molecules. Thus, an array of different lectins is needed to identify the presence of different carbohydrates in a given sample (6). This can be defined as selective sensing. An example of specific sensing is the application of Glucose oxidase (GOx) in blood glucose monitoring. Glucose oxidase binds specifically to glucose, and no other sugar or analyte. This ensures that when a drop of blood is used as a sample, the signal produced from the transducer in the sensor, is generated from the presence of glucose only. Therefore, this can be defined as specific sensing.

Importance of the concepts

Regardless of the technology that is applied, selectivity and specificity are a vital component of any biological assay. They allow for ‘true results’ to be distinguished from a sample, eliminating any background noise, that might otherwise provide a different outcome (7). Continuous advancements in technology that would allow us to have higher selectivity and specificity in biological assays, would provide more accurate results and ultimately lead to a more optimum healthcare.

References:

1. Recent advances and challenges in electrochemical biosensors for emerging and re-emerging infectious diseases. Menon, S, et al. s.l. : Journal of Electroanalytical Chemistry, 2020, Vol. 878.

2. Grand View Research. Biosensors Market Size, Share & Trends Analysis Report By Application (Medical, Agriculture, Environment) By Technology (Thermal, Electrochemical, Optical), By End-use, By Region, And Segment Forecasts, 2021 - 2028 . 2020.

3. Selectivity and specificity in analytical chemistry. General considerations and attempt of a definition and quantification. Danzer, Klaus. s.l. : Journal of Analytical Chemistry, 2001, Vol. 369.

4. Selectivity and Specificity: Pros and Cons in Sensing. Peveler, W J, Yazdani, M and Rotello, V M. 11, s.l. : ACS Sensors, 2016, Vol. 1.

5. Plasma protein biosignatures for detection of cardiac allograft vasculopathy. David, Lin, et al. 7, s.l. : The Journal of heart and lung transplantation, 2013, Vol. 32.

6. Lectin microarrays: concept, principle and applications. Hirabayashi, Jun, et al. 10, s.l. : Chemical Society Reviews , 2013, Vol. 42.

7. SELECTIVITY IN ANALYTICAL CHEMISTRY (IUPAC Recommendations 2001). JÖRGEN VESSMAN, RALUCA I. STEFAN, JACOBUS F. VAN STADEN2, KLAUS DANZER, WOLFGANG LINDNER, DUNCAN THORBURN BURNS, ALES FAJGELJ, HELMUT MÜLLER. 8, s.l. : Pure Applied Chemistry, 2001, Vol. 73.