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.

We describe the identification of bicyclic RGD peptides with high affinity and selectivity for integrin α v β 3 via high-throughput screening of partially randomized libraries. Peptide libraries (672 different compounds) comprising the universal integrin-binding sequence Arg-Gly-Asp (RGD) in the first loop and a randomized sequence XXX (X being one of 18 canonical L-amino acids) in the second loop, both enclosed by either an L-or D-Cys residue, were converted to bicyclic peptides via reaction with 1,3,5-tris(bromomethyl)benzene (T3). Screening of first-generation libraries yielded lead bicyclic inhibitors displaying submicromolar affinities for integrin α v β 3 (e.g., C T3 HEQc T3 RGDc T3 , IC 50 = 195 nM). Next generation (second and third) libraries were obtained by partially varying the structure of the strongest lead inhibitors and screening for improved affinities and selectivities. In this way, we identified the highly selective bicyclic α v β 3 -binders C T3 HPQc T3 RGDc T3 (IC 50 = 30 nM), C T3 HPQC T3 RGDc T3 (IC 50 = 31 nM), and C T3 HSQC T3 RGDc T3 (IC 50 = 42 nM) with affinities comparable to that of a knottin-RGD-type peptide (32 amino acids, IC 50 = 38 nM) and outstanding selectivities over integrins α v β 5 (IC 50 > 10000 nM) and α 5 β 1 (IC 50 > 10000 nM). Affinity measurements using surface plasmon-enhanced fluorescence spectroscopy (SPFS) yielded K d values of 0.4 and 0.6 nM for the Cy5-labeled bicycle C T3 HPQc T3 RGDc T3 and RGD "knottin" peptide, respectively. In vitro staining of HT29 cells with Cy5labeled bicycles using confocal microscopy revealed strong binding to integrins in their natural environment, which highlights the high potential of these peptides as markers of integrin expression.

We report the identification of high-affinity and selectivity integrin α5β1-binding bicyclic peptides via “designed random libraries”, that is, the screening of libraries comprising the universal integrin-binding sequence Arg-Gly-Asp (RGD) in the first loop in combination with a randomized sequence (XXX) in the second loop. Screening of first-generation libraries for α5β1-binding peptides yielded a triple-digit nanomolar bicyclic α5β1-binder (C T3 RGDc T3 AYGC T3 , IC50 = 406 nM). Next-generation libraries were designed by partially varying the structure of the strongest first-generation lead inhibitor and screened for improved affinities and selectivities for this receptor. In this way, we identified three high-affinity α5β1-binders (C T3 RGDc T3 AYJC T3 , J = d-Leu, IC50 = 90 nM; C T3 RGDc T3 AYaC T3 , IC50 = 156 nM; C T3 RGDc T3 AWGC T3 , IC50 = 173 nM), of which one even showed a higher α5β1-affinity than the 32 amino acid benchmark peptide knottin-RGD (IC50 = 114 nM). Affinity for α5β1-integrin was confirmed by SPFS analysis showing a K d of 4.1 nM for Cy5-labeled RGD-bicycle C T3 RGDc T3 AYJC T3 (J = d-Leu) and a somewhat higher K d (9.0 nM) for Cy5-labeled knottin-RGD. The α5β1-bicycles, for example, C T3 RGDc T3 AYJC T3 (J = d-Leu), showed excellent selectivities over αvβ5 (IC50 ratio α5β1/αvβ5 between <0.009 and 0.039) and acceptable selectivities over αvβ3 (IC50 ratios α5β1/αvβ3 between 0.090 and 0.157). In vitro staining of adipose-derived stem cells with Cy5-labeled peptides using confocal microscopy revealed strong binding of the α5β1-selective bicycle C T3 RGDc T3 AWGC T3 to integrins in their natural environment, illustrating the high potential of these RGD bicycles as markers for α5β1-integrin expression.

A combined approach to signal enhancement in fluorescence affinity biosensors and assays is reported. It is based on the compaction of specifically captured target molecules at the sensor surface followed by optical probing with a tightly confined surface plasmon (SP) field. This concept is utilized by using a thermoresponsive hydrogel (HG) binding matrix that is prepared from a terpolymer derived from poly­(N-isopropylacrylamide) (pNIPAAm) and attached to a metallic sensor surface. Epi-illumination fluorescence and SP-enhanced total internal reflection fluorescence readouts of affinity binding events are performed to spatially interrogate the fluorescent signal in the direction parallel and perpendicular to the sensor surface. The pNIPAAm-based HG binding matrix is arranged in arrays of sensing spots and employed for the specific detection of human IgG antibodies against the Epstein–Barr virus (EBV). The detection is performed in diluted human plasma or with isolated human IgG by using a set of peptide ligands mapping the epitope of the EBV nuclear antigen. Alkyne-terminated peptides were covalently coupled to the pNIPAAm-based HG carrying azide moieties. Importantly, using such low-molecular-weight ligands allowed preserving the thermoresponsive properties of the pNIPAAm-based architecture, which was not possible for amine coupling of regular antibodies that have a higher molecular weight.

Strategies to develop antifouling surface coatings are crucial for surface plasmon resonance (SPR) sensing in many analytical application fields, such as detecting human disease biomarkers for clinical diagnostics and monitoring foodborne pathogens and toxins involved in food quality control. In this review, firstly, we provide a brief discussion with considerations about the importance of adopting appropriate antifouling materials for achieving excellent performances in biosensing for food safety and clinical diagnosis. Secondly, a non-exhaustive landscape of polymeric layers is given in the context of surface modification and the mechanism of fouling resistance. Finally, we present an overview of some selected developments in SPR sensing, emphasizing applications of antifouling materials and progress to overcome the challenges related to the detection of targets in complex matrices relevant for diagnosis and food biosensing.