Aptasensors versus immunosensors-Which will prevail?

Abstract

Since the invention of the first biosensors 70 years ago, they have turned into valuable and versatile tools for various applications, ranging from disease diagnosis to environmental monitoring. Traditionally, antibodies have been employed as the capture probes in most biosensors, owing to their innate ability to bind their target with high affinity and specificity, and are still considered as the gold standard. Yet, the resulting immunosensors often suffer from considerable limitations, which are mainly ascribed to the antibody size, conjugation chemistry, stability, and costs. Over the past decade, aptamers have emerged as promising alternative capture probes presenting some advantages over existing constraints of immunosensors, as well as new biosensing concepts. Herein, we review the employment of antibodies and aptamers as capture probes in biosensing platforms, addressing the main aspects of biosensor design and mechanism. We also aim to compare both capture probe classes from theoretical and experimental perspectives. Yet, we highlight that such comparisons are not straightforward, and these two families of capture probes should not be necessarily perceived as competing but rather as complementary. We, thus, elaborate on their combined use in hybrid biosensing schemes benefiting from the advantages of each biorecognition element.

Keywords: antibody; antibody‐aptamer hybrid; aptamer; biosensors; capture probes.

FIGURE 1

Antibodies as capture probes in immunosensors. (A) Schematic overview detailing the structure of whole antibodies and their fragments such as Fab, Fab’, scAb and scFv. (B) Strategies for immobilization of antibodies and their derivatives onto biosensor surfaces. These strategies include various covalent and non‐covalent approaches for random and oriented biosensor functionalization

FIGURE 2

Illustration of the 3D folding of aptamers and their binding to their target analyte

FIGURE 3

Immunosensor and aptasensor biosensing concepts. (A) Immunosensors are often used in conventional sensing formats. (i) In direct assays, the target analyte binds to the capture probe, which is immobilized onto the sensor surface, while in sandwich assays (ii) a secondary antibody is used to increase the selectivity and sensitivity of the assay. (iii) In competitive assays, an artificially‐labeled analyte and the “real” target analyte compete for the capture probe binding sites. (B) While aptamers can also be used in such conventional sensing schemes, more elaborated concepts have been developed. (i) Target binding induces conformational changes of the aptamer, which is labeled with a redox‐active molecule, altering electron transfer (eT) and thus the measurable signal. (ii) The binding of split aptamers upon analyte binding causes a detectable signal. For instance, one aptamer that is labeled with a quencher molecule represses the fluorescence of the second fluorescently labeled aptamer. (iii) Target binding can cause a complementary DNA strand to dissociate that is subsequently used as a primer in an RCA reaction to increase the sensitivity

FIGURE 4

Hybrid biosensors employing aptamers and antibodies as capture probes. (A) Sensitivity enhancement by application of an antibody as a secondary capture probe for detection of protein A with an aptamer‐functionalized porous silicon biosensor. Reproduced with permission from [169]. Copyright 2017, Elsevier B.V. (B) Schematic illustration of the RCA based hetero‐sandwich electrochemical biosensor for ultrasensitive detection of Vibrio parahaemolyticus. Reproduced with permission from [166]. Copyright 2017, Springer Nature. (C‐i) Schematic illustration of an aptamer‐antibody dual probe system for amyloid‐beta (AβO) determination, based on a nanohorn‐modified dielectrode. The dual probe system was compared to a single antibody probe (top panel), as well as to a single step (bottom panel) and a two‐step (middle panel) immobilization routes of both capture probes. (C‐ii) Comparison of biosensing signal for detection of amyloid‐beta with the different platforms, presenting improved performance for the dual probe scheme. Adapted with permission from [176]. Copyright 2021, the authors. Published under Attribution‐NonCommercial 3.0 Unported (CC BY‐NC 3.0)