Nucleic acid biomarkers are central to modern healthcare, supporting early diagnosis and personalised treatment decisions. At the same time, advances in silicon-based microtechnologies are enabling mass-manufacturable photonic sensors that are compact, multiplexed and suitable for point-of-care applications. A major challenge remains the reproducible control of DNA probe density on silicon, which critically influences hybridisation efficiency and overall biosensor performance. Many conventional silanisation routes are sensitive to reaction conditions and often require multi-step, time-intensive protocols, making it difficult to create functional DNA surfaces with minimal crowding and steric hindrance effects. Here, we introduce a tuneable, one-step PEG-silanisation strategy that offers a practical route to forming functional monolayers with controllable functional-group density on silicon substrates. By adjusting the ratio of functional to non-functional PEG-silanes, we achieve robust and reproducible modulation of DNA probe grafting density, enhancing probe immobilisation and improving target hybridisation performance. Surface composition and molecular arrangement were characterised using standard and angle-resolved X-ray photoelectron spectroscopy, atomic force microscopy and ellipsometry. Efficiency in DNA probe immobilisation and target hybridisation was verified using fluorescence-labelled biomolecules, revealing a strong dependence on molecular crowding at both the DNA probe immobilisation and target levels, yielding up to ten-fold enhancement in hybridisation efficiency. The method is fully compatible with silicon photonic platforms, as demonstrated using asymmetric Mach-Zehnder interferometers. Overall, this approach provides a standardisable, practical framework for engineering DNA-functionalised surfaces on silicon, suitable for label-free biosensing applications.

Personalized immunotherapies hold great promise for correcting cellular dysfunction, inhibiting tumor growth, and even achieving durable cancer eradication. However, conventional analytical methods used to design and evaluate immunotherapies often fall short as they typically cannot monitor cell activity in real time, lack multiplexed capabilities for rapid screening, and require complex sample preparation. These limitations impede a full understanding of the dynamic immune responses that drive therapeutic outcomes. Label-free optical biosensors based on evanescent wave interactions provide a compelling alternative, enabling sensitive, noninvasive, and high-resolution analysis of cellular regulation, signaling, and therapy-induced molecular changes. In this perspective, we highlight recent advances in optical biosensor technologies for molecular and cell analysis and explore their potential to accelerate the development, optimization, and precision application of next-generation immunotherapies.

Accurate diagnosis of myocardial injury is crucial in clinical practice, as it enables prompt treatment, reduces the risk of complications, and improves patient outcomes. Examining cardiac troponin composition offers distinct advantages as it provides valuable insights that help differentiate between ischaemic and non-ischaemic causes of cardiac injury, thereby directly enhancing clinical assay specificity. Current high-sensitivity troponin assays target all circulating forms of either troponin I or T but are unable to distinguish individual forms or specific combinations. This is an issue as troponin is also elevated in non-cardiac conditions, and emerging evidence suggests that the ratio between troponin I and TIC complex isoforms may serve as an indicator for differentiating acute from chronic myocardial injury. Here, we introduce a methodology utilising a label-free Surface Plasmon Resonance biosensor to detect and differentiate between troponin I and complex isoforms. Unlike traditional immunoassay, which requires indirect labelling with secondary antibodies to detect these biomarkers, this method provides a real-time detection of these isoforms, as each antibody interaction with the specific analytes provides a quantitative reading. We demonstrated that this method can detect complexed and free troponin I in varying ratios. This method has the potential to be developed into point-of-care testing and opens avenues for isoform-specific prognostic assays.

Silicon photonic microring resonators have emerged as promising sensors for Point‐of‐Care applications, where the readout of one or many biomarkers at once is required. In the context of rapid heart attack detection, a limit of detection reaching an ultra‐low concentration of biomarkers is needed, however, such sensors are prone to fundamental noise influence in optical systems which can potentially jeopardize sensor readings. While noise reduction has previously been explored with the Pound–Drever–Hall (PDH) technique, its full implementation in microring biosensors has not been realized due to the complexity of the setup. Recent innovations in photonic integration and compatibility with MEMS structures have sparked new interest in validating PDH's potential to be used with chip‐scale sensors. By enabling the signal readout of microrings through their phase response, instead of power transmission, the impact of optical noise can be greatly reduced. This study explores a proof‐of‐concept for this system against the cardiac troponin biomarker, demonstrating the sensor's capacity for selective measurement down to a limit of 10 ng mL −1 while using frequency locking. An improved limit of detection for this system is achieved, down to 5.03 × 10 −7 RIU, which is two orders of magnitude improved compared to the equivalent sensing based on intensity alone.

For early diabetes identification and management, the progression of an uncomplicated and exceedingly responsive glucose testing technology is crucial. In this study, we present a new sensor incorporating a composite of metal organic framework (MOF) based on cobalt, coated with boronic acid to facilitate selective glucose binding. Additionally, we successfully employed a highly sensitive electro-optical immunosensor for the detection of subtle changes in concentration of the diabetes biomarker glycated haemoglobin (HbA1c), using zeolitic imidazolate framework-67 (ZIF-67) coated with polydopamine which further modified with boronic acid. Utilizing the polymerization characteristics of dopamine and the NH2 groups, a bonding structure is formed between ZIF-67 and 4-carboxyphenylboronic acid. ZIF-67 composite served as an effective substrate for immobilising 4-carboxyphenylboronic acid binding agent, ensuring precise and highly selective glucose identification. The sensing response was evaluated through both electrochemical and optical methods, confirming its efficacy. Under optimized experimental condition, the ZIF-67 based sensor demonstrated a broad detection range of 50–500 mg dL−1, a low limit of detection (LOD) of 9.87 mg dL−1 and a high correlation coefficient of 0.98. Furthermore, the 4-carboxyphenylboronic acid-conjugated ZIF-67-based sensor platform exhibited remarkable sensitivity and selectivity in optical-based detection for glycated haemoglobin within the clinical range of 4.7–11.3

Glycosylated hemoglobin (HbA1c) is a relevant analyte to monitor the efficiency of a diabetic patient's treatment regime. The development of a simple and sensitive glucose-testing method is critical for early diabetes diagnosis and treatment. In order to address this challenge, a sensitive electrochemical-optical immunosensor has been successfully fabricated utilizing zeolitic imidazolate framework-8 nanoparticles (NPs) (ZIF-8) doped with polydopamine for subtle exploration of diabetes biomarker HbA1c. Density functional theory is used to study glucose adsorption on boronic-acid (BA)-conjugated ZIF-8 NPs. Such analyses indicated that the BA over the ZIF-8 metal-organic framework greatly favored glucose adsorption on the surface. The adsorption of the glucose molecule is accomplished through the transfer of electrons from the conjugate to the glucose molecule, which enhances the sensing response. The polymerization nature of dopamine and the presence of NH2 groups are harnessed to make a bonding structure between ZIF-8 and 4-carboxyphenylboronic acid. Furthermore, under optimal experimental conditions, the developed carboxyphenylboronic acid-conjugated ZIF-8-based sensor has shown responses within the range of 100-450 mg/dL glucose concentration through electrochemical studies. A dual-sensing technique using optical- and electrochemical-based detection was used for the sensing of HbA1c. The sensor was able to detect HbA1c in whole blood by an optical detection method within the clinically relevant range of 4.8-12.7% of HbA1c with a low limit of detection of 3.4% and a correlation coefficient of 0.98. The combined computational and experimental studies have shown that the ZIF-8 NP-based sensing matrix is a potential candidate for the nonenzymatic detection of glucose and HbA1c.

Plasmonic biosensors are very versatile and powerful technologies able to analyze, detect, and quantify virtually any biological entity in a label-free and real-time format. Such capabilities make them unique tools for improving and upgrading current medical diagnosis, promoting the point-of-care testing for early disease detection and large-scale population screening. In this chapter, we provide a comprehensive revision of the most common and promising applications of plasmonic biosensors in clinics, including diagnosis of cancer, pathogenic infections, cardiovascular diseases, and other serious health disorders. Current status and major prospects of these diagnostic biosensors are critically discussed from a clinical implementation perspective.

For the characterization of biological interactions, plasmonic immunosensors have come to the forefront in clinical settings specifically for analysis of various pathological markers. Surface functionalization is a key step for efficient antibody immobilization while limiting aberrant non-specific interactions. Various surface chemistry modalities have been used from affinity based such as avidin/biotin and protein A/G to covalent based carbodiimide/NHS coupling. However, these immobilization modalities have their limitations such as complex modification steps, long-term monolayer stability and background specificity. Strain promoted azide alkyne cycloaddition (SPAAC) is a covalent based modality that allows conjugation of specific biomolecules in an oriented manner. Dibenzocyclooctyne (DBCO) based SPAAC reactions provides avenue for two different attachment approaches either through thiol-yne reaction or initiation of triazole formation with an azide group. Here, we have conducted an oriented and site-specific antibody immobilization strategies through both thiol group of Fab fragmented antibody and azide modification of glycosylated part of antibody for directed coupling on DBCO functionalized sensors. Conventional surface plasmon resonance imaging setup was employed for characterization of sensing efficiencies of these modified immobilized antibodies. In addition, performance of alkylated and polyethylene glycol (PEG)-ylated DBCO functionalized sensors have been showcased along with comparative analysis with the conventional covalent carbodiimide/NHS strategy.

High Tumor Necrosis Factor Receptor 2 (TNFR2) expression is characteristic of diverse malignant cells during tumorigenesis. The protein is also expressed by many immunosuppressive cells during cancer development, allowing cancer immune escape. A growing body of evidence further suggests a correlation between the circulating form of this protein and cancer development. Here we conducted a systematic meta-analysis of cancer studies published up until 1 st October 2022, in which the circulating soluble TNFR2 (sTNFR2) concentrations in patients with cancers were recorded and their association with cancer risk was assessed. Of the 14,615 identified articles, 44 studies provided data on the correlation between cancer risk and the level of circulating sTNFR2. The pooled means comparison showed a consistently significant increase in the levels of sTNFR2 in diverse cancers when compared to healthy controls. These included colorectal cancer, ovarian cancer, breast cancer, non-Hodgkin’s lymphoma, Hodgkin’s lymphoma, lung cancer, hepatocarcinoma, and glioblastoma. In a random-effect meta-analysis, the cancer-specific odd ratios (OR) showed significant correlations between increased circulating sTNFR2 levels and the risk of colorectal cancer, non-Hodgkin’s lymphoma, and hepatocarcinoma at 1.59 (95% CI:1.20-2.11), 1.98 (95% CI:1.49-2.64) and 4.32 (95% CI:2.25-8.31) respectively. The overall result showed an association between circulating levels of sTNFR2 and the risk of developing cancer at 1.76 (95% CI:1.53-2.02). This meta-analysis supports sTNFR2 as a potential diagnostic biomarker for cancer, albeit with different predictive strengths for different cancer types. This is consistent with a potential key role for TNFR2 involvement in cancer development.

Cancer is one of the leading cause of death worldwide. Lung cancer (LCa) and prostate cancer (PCa) are the two most common ones particularly among men with about 20% of aggressive metastatic form leading to shorter overall survival. In recent years, circulating tumor cells (CTCs) have been investigated extensively for their role in metastatic progression and their involvement in reduced overall survival and treatment responses. Analysis of these cells and their associated biomarkers as "liquid biopsy" can provide valuable real-time information regarding the disease state and can be a potential avenue for early-stage detection and possible selection of personalized treatments. This review focuses on the role of CTCs and their associated biomarkers in lung and prostate cancer, as well as the shortcomings of conventional methods for their isolation and analysis. To overcome these drawbacks, biosensors are an elegant alternative because they are capable of providing valuable multiplexed information in real-time and analyzing biomarkers at lower concentrations. A comparative analysis of different transducing elements specific for the analysis of cancer cell and cancer biomarkers have been compiled in this review.

Chronic inflammation generated by the tumor microenvironment is known to drive cancer initiation, proliferation, progression, metastasis, and therapeutic resistance. The tumor microenvironment promotes the secretion of diverse cytokines, in different types and stages of cancers. These cytokines may inhibit tumor development but alternatively may contribute to chronic inflammation that supports tumor growth in both autocrine and paracrine manners and have been linked to poor cancer outcomes. Such distinct sets of cytokines from the tumor microenvironment can be detected in the circulation and are thus potentially useful as biomarkers to detect cancers, predict disease outcomes and manage therapeutic choices. Indeed, analyses of circulating cytokines in combination with cancer-specific biomarkers have been proposed to simplify and improve cancer detection and prognosis, especially from minimally-invasive liquid biopsies, such as blood. Additionally, the cytokine signaling signatures of the peripheral immune cells, even from patients with localized tumors, are recently found altered in cancer, and may also prove applicable as cancer biomarkers. Here we review cytokines induced by the tumor microenvironment, their roles in various stages of cancer development, and their potential use in diagnostics and prognostics. We further discuss the established and emerging diagnostic approaches that can be used to detect cancers from liquid biopsies, and additionally the technological advancement required for their use in clinical settings.

The bimodal waveguide (BiMW) biosensor is an innovative common path interferometric sensor based on the evanescent field detection principle. This biosensor allows for the direct detection of virtually any biomolecular interaction in a label-free scheme by using specific biorecognition elements. Due to its inherent ultrasensitivity, it has been employed for the monitoring of relevant nucleic-acid sequences such as mRNA transcripts or microRNAs present at the attomolar-femtomolar concentration level in human samples. The application of the BiMW biosensor to detect these nucleic acids can be a powerful analytical tool for diagnosis and prognosis of complex illnesses, such as cancer, where these biomarkers play a major role. The BiMW sensor is fabricated using standard silicon-based microelectronics technology, which allows its miniaturization and cost-effective production, meeting the requirements of portability and disposability for the development of point-of-care (PoC) sensing platforms.In this chapter, we describe the working principle of the BiMW biosensor as well as its application for the analysis of nucleic acids. Concretely, we show a detailed description of DNA functionalization procedures and the complete analysis of two different RNA biomarkers for cancer diagnosis: (1) the analysis of mRNA transcripts generated by alternative splicing of Fas gene, and (2) the detection of miRNA 181a from urine liquid biopsies, for the early diagnosis of bladder cancer. The biosensing detection is performed by a direct assay in real time, by monitoring the changes in the intensity pattern of the light propagating through the BiMW biosensor, due to the hybridization of the target with the specific DNA probe previously functionalized on the BiMW sensor surface.

Label-free plasmonic biosensors have demonstrated promising capabilities as analytical tools for the detection of virtually any type of biomarker. They are presented as good candidates for precision diagnostics since they offer highly sensitive, cost-effective solutions that can be used in any clinical or laboratory setting without the need for specialized trainees. However, different surface functionalization protocols are required, depending on the nature of the biorecognition element, limiting their capabilities for integrated multi-biomarker detection. Here, we present a simple, yet efficient, one-step immobilization approach that is common for both DNA probes and antibodies. Our immobilization approach relies on the incorporation of poly-adenine (polyA) blocks in both nucleic acid probes and antibodies. PolyA sequences have a remarkable affinity for gold surfaces and can specifically interact with sufficient strength to generate stable, dense, and highly ordered monolayers. We have demonstrated excellent performance of our universal functionalization method, showing limits of detection and quantification in the pM-nM range. Moreover, it was able to reduce up to 50% of the background signal from undiluted serum samples compared to conventional methods, demonstrating the immense potential of this strategy for the direct analysis of human biofluids, essential for rapid point-of-care diagnostics. The polyA-based immobilization approach is a promising alternative for the generation of multiplexed biosensors that can detect both protein and nucleic acid biomarkers for multiparametric diagnostic assays.

Pneumocystis jirovecii is a fungus responsible for human Pneumocystis pneumonia, one of the most severe infections encountered in immunodepressed individuals. The diagnosis of Pneumocystis pneumonia continues to be challenging due to the absence of specific symptoms in infected patients. Moreover, the standard diagnostic method employed for its diagnosis involves mainly PCR-based techniques, which besides being highly specific and sensitive, require specialized personnel and equipment and are time-consuming. Our aim is to demonstrate an optical biosensor methodology based on surface plasmon resonance to perform such diagnostics in an efficient and decentralized scheme. The biosensor methodology employs poly-purine reverse-Hoogsteen hairpin probes for the detection of the mitochondrial large subunit ribosomal RNA (mtLSU rRNA) gene, related to P. jirovecii detection. The biosensor device performs a real-time and label-free identification of the mtLSU rRNA gene with excellent selectivity and reproducibility, achieving limits of detection of around 2.11 nM. A preliminary evaluation of clinical samples showed rapid, label-free and specific identification of P. jirovecii in human lung fluids such as bronchoalveolar lavages or nasopharyngeal aspirates. These results offer a door for the future deployment of a sensitive diagnostic tool for fast, direct and selective detection of Pneumocystis pneumonia disease.

In this contribution, we demonstrate how an optical frequency comb can be used to enhance the functionality of an integrated photonic biosensor platform. We show that if an optical frequency comb is used to sample the spectral response of a Mach-Zehnder interferometer and if the line spacing is arranged to sample the periodic response at 120° intervals, then it is possible to combine these samples into a single measurement of the interferometer phase. This phase measurement approach is accurate, independent of the bias of the interferometer and robust against intensity fluctuations that are common to each of the comb lines. We demonstrate this approach with a simple silicon photonic interferometric refractive index sensor and show that the benefits of our approach can be obtained without degrading the lower limit of detection of 3.70×10-7 RIU.

Dual frequency combs are used to extract a linear phase response of interferometric biosensors while being independent of the bias point and offering common-mode rejection and low noise.

We report a silicon Mach-Zehnder interferometer biosensor with an integrated microfluidic sample handling for an accurate and timely detection of cardiac troponin. The performance of the photonic biosensor was evaluated in terms of sensitivity, selectivity and reproducibility following the international clinical guidelines for acute myocardial infarction with the obtention of a complete cardiac troponin point-of-care test. We demonstrated that this biosensor was able to selectively detect cardiac troponin within 10 minutes in the ng/mL-μg/mL range with high reproducibility, achieving a limit of detection as low as 3 ng/mL in a direct assay.

Introduction: Optical biosensors, particularly those based on nanoplasmonics technology, have emerged in recent decades as a potential solution for disease diagnostics and therapy follow-up at the point-of-care (POC). These biosensor platforms could overcome some of the challenges faced in conventional diagnosis techniques offering label-free assays with immediate results and employing small and user-friendly devices. Areas covered: In this review, we will provide a critical overview of the recent advances in the development of nanoplasmonic biosensors for the POC diagnostics. We focus on those systems with demonstrated capabilities for integration in portable platforms, highlighting some of the most relevant diagnostics applications targeting proteins, nucleic acids, and cells as disease biomarkers. Expert commentary: Despite the attractive features of label-free nanoplasmonic sensors in terms of miniaturization and analytical robustness, the route toward an effective clinical implementation involves the integration of fully automated microfluidic systems for sample processing and analysis, and the optimization of surface biofunctionalization procedures. Additionally, the development of multiplexed sensors for high-throughput analysis and including specific neoantigens and novel biomarkers in detection panels will provide the means for delivering a powerful analytical technology for an accurate and improved medical diagnosis.

Silicon photonics has demonstrated great potential in ultrasensitive biochemical sensing. However, it is challenging for such sensors to detect small ions which are also of great importance in many biochemical processes. A silicon photonic ion sensor enabled by an ionic dopant-driven plasmonic material is introduced here. The sensor consists of a microring resonator (MRR) coupled with a 2D restacked layer of near-infrared plasmonic molybdenum oxide. When the 2D plasmonic layer interacts with ions from the environment, a strong change in the refractive index results in a shift in the MRR resonance wavelength and simultaneously the alteration of plasmonic absorption leads to the modulation of MRR transmission power, hence generating dual sensing outputs which is unique to other optical ion sensors. Proof-of-concept via a pH sensing model is demonstrated, showing up to 7 orders improvement in sensitivity per unit area across the range from 1 to 13 compared to those of other optical pH sensors. This platform offers the unique potential for ultrasensitive and robust measurement of changes in ionic environment, generating new modalities for on-chip chemical sensors in the micro/nanoscale.