Biomarker-based cancer analysis has great potential to lead to a better understanding of disease at the molecular level and to improve early diagnosis and monitoring. Unlike conventional tissue biopsy, liquid biopsy allows the detection of a large variety of circulating biomarkers, such as microRNA (miRNA), exosomes, circulating tumor DNA (ctDNA), circulating tumor cells (CTCs), and proteins, in an easily accessible and minimally invasive way. In this review, we describe and evaluate the relevance and applicability of surface plasmon resonance (SPR) and localized SPR (LSPR)-based platforms for the detection of different classes of cancer biomarkers in liquid biopsy samples. Firstly, we critically discuss unsolved problems and issues in capturing and analyzing biomarkers. Secondly, we highlight current challenges which need to be resolved in applying SPR biosensors into clinical practice. Then, we mainly focus on applications of SPR-based platforms that process a patient sample aiming to detect and quantify biomarkers as a minimally invasive liquid biopsy tool for cancer patients appearing over the last 5 years. Finally, we describe the analytical performances of selected SPR biosensor assays and their significant advantages in terms of high sensitivity and specificity as well as accuracy and workflow simplicity.

A major challenge in the area of DNA detection is the development of rapid methods that do not require polymerase chain reaction (PCR) amplification of the genetic sample. The PCR amplification step increases the cost of the assay, the complexity of the detection, and the quantity of DNA required for the assay. In this context, methods that are able to perform DNA analyses with ultrasensitivity have recently been investigated with the aim of developing new PCR-free detection protocols. Functionalized gold nanoparticles have played a central role in the development of such methods. Here, possibilities offered by functionalized gold nanoparticle in the ultrasensitive detection of DNA are discussed. The different functionalization protocols available for gold nanoparticles and the principal DNA detection methods that are able to detect DNA at the femtomolar to attomolar level are presented.

Organic electronic transistors are rapidly emerging as ultra-high sensitive label-free biosensors suited for point of care or in-field deployed applications. Most organic biosensors reported to date are based on immunorecognition between the relevant biomarkers and the immobilized antibodies, whose use is hindered by large dimensions, poor control of sequence and relative instability. Here, we report an Electrolyte Gated Organic Field Effect Transistor (EGOFET) biosensor where the recognition units are surface immobilized peptide aptamers (Affimerä proteins) instead of antibodies. We demonstrate our peptide aptasensor for the detection of the pro-inflammatory cytokine Tumor Necrosis Factor alpha (TNFa) with a 1pM limit of detection. Ultra-low sensitivity is met even in complex solutions such as cell culture media containing 10 % serum, demonstrating the remarkable ligand specificity of our device. The device performances, together with the simple one-step immobilization strategy of the recognition moieties and the low operational voltages, all prompt EGOFET peptide aptasensors as candidates for early diagnostics and monitoring at the point-of-care.

The monitoring of DNA and RNA biomarkers freely circulating in the blood constitutes the basis of innovative cancer detection methods based on liquid biopsy. Such methods are expected to provide new opportunities for a better understanding of cancer disease at the molecular level, thus contributing to improved patient outcomes. Advanced biosensors can advance possibilities for cancer-related nucleic acid biomarkers detection. In this context, peptide nucleic acids (PNAs) play an important role in the fabrication of highly sensitive biosensors. This review provides an overview of recently described PNA-based biosensors for cancer biomarker detection. One of the most striking features of the described detection approaches is represented by the possibility to detect target nucleic acids at the ultra-low concentration with the capability to identify single-base mutations.

To explore the variability in biosensor studies, 150 participants from 20 countries were given the same protein samples and asked to determine kinetic rate constants for the interaction. We chose a protein system that was amenable to analysis using different biosensor platforms as well as by users of different expertise levels. The two proteins (a 50-kDa Fab and a 60-kDa glutathione S-transferase [GST] antigen) form a relatively high-affinity complex, so participants needed to optimize several experimental parameters, including ligand immobilization and regeneration conditions as well as analyte concentrations and injection/dissociation times. Although most participants collected binding responses that could be fit to yield kinetic parameters, the quality of a few data sets could have been improved by optimizing the assay design. Once these outliers were removed, the average reported affinity across the remaining panel of participants was 620 pM with a standard deviation of 980 pM. These results demonstrate that when this biosensor assay was designed and executed appropriately, the reported rate constants were consistent, and independent of which protein was immobilized and which biosensor was used.

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Ultrasensitive detection protocols not requiring polymerase chain reaction (PCR)-mediated target DNA amplification are expected to significantly improve our possibilities in several research and diagnostic applications for which minute cell quantities are available. For this reason we have tested a nanoparticle-enhanced surface plasmon resonance imaging (SPRI) sensing strategy to detect point mutations in nonamplified genomic DNA. We have used genomic DNAs, not subject to costly, time-consuming, and prone to contamination PCR-based amplification procedures, obtained from both healthy individuals and homozygous or heterozygous patients affected by β-thalassemia, in order to demonstrate the specificity and the sensitivity of the described sensing strategy. The assay we describe is ultrasensitive and convenient. Attomolar concentrations of target genomic DNA are detected, DNAs from healthy individuals and homozygous or heterozygous patients affected by β-thalassemia are discriminated, and only simple manipulations of the genetic samples are required before the analysis. The proposed ultrasensitive detection of DNA point mutations involved in genomic disorders possibly represents an important advantage in several biomedical applications.

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