Direct optical detection in bioanalysis: an update

Dual-polarization interferometry for quantification of small molecules using aptamers

An interferometry-based method was developed for detection of a small molecule, argininamide. The quantification of argininamide was demonstrated using aptamers immobilized on silicone oxynitride sensor surface via avidin-biotin binding. The aptamers formed a thin film over avidin layer corresponding to a thickness of 1.2 nm, consistent with a molecular positioning of multipoint attachment to the surface. The binding of argininamide did not cause any significant changes in the thickness of the aptamer film, suggesting that the specific binding did not affect the overall conformation of the aptamer molecules after adaptive rearrangement of aptamer molecules. However, the binding results in clearly detectable changes in mass calculated from multiple parameters determined by mass deposition and structural changes. The limit of detection of the developed sensor was determined to be 5 μM. The sensor can monitor real-time changes in argininamide concentrations with high reliability and sensitivity. The model system demonstrated that a combined measurement considering structural and mass changes through interferometry-based techniques can overcome one of the major problems associated with real-time monitoring of small mass analytes.

Reflectometric interference spectroscopy (RIfS) as a new tool to measure in the complex matrix milk at low analyte concentration

Measurements in complex matrices like milk still present a challenge in biosensor development. This is especially important when using a label-free detection method or when measuring low analyte concentrations. The direct optical method reflectometric interference spectroscopy (RIfS) was used for investigating matrix effects in immunoassay development. Furthermore, approaches to reduce these effects have been established. As a model system, the hormone testosterone has been chosen because this immunoassay has been well characterized in buffer. In a first step, the immunoassay for the detection of testosterone in buffer was improved beyond former published results. Therefore, the sensor surface was optimized, resulting in a fivefold lower limit of detection (70.2 ng L(-1)) and limit of quantification (130.0 ng L(-1)). Additionally, the assay time could be reduced to 15 min. Consequently, we used this improved assay to investigate matrix effects of whole pasteurized bovine milk. To minimize these effects, the surface chemistry was adapted and a suitable evaluation method was established, reducing the effects of Tyndall scattering and nonspecific binding to the sensor surface. These improvements allow for very reliable quantitative measurements in milk. The assay developed required no sample pretreatment and allowed for the regeneration of the sensor surface so that calibration could be performed on one chip. The calibration in milk (3.5% fat) resulted in a limit of detection of 94.4 ng L(-1) and a limit of quantification of 229.3 ng L(-1). Furthermore, recovery rates between 70% and 120% could be obtained. Thus, for the first time, an analyte in the matrix milk was successfully quantified with RIfS at low concentrations.

Enhancing sensitivity and selectivity of long-period grating sensors using structure-switching aptamers bound to gold-doped macroporous silica coatings

High surface area, sol-gel derived macroporous silica films doped with gold nanoparticles (AuNP) are used as a platform for high-density affinity-based immobilization of functional structure-switching DNA aptamer molecules onto Michelson interferometer long-period grating (LPG) fiber sensors, allowing for label-free detection of small molecular weight analytes such as adenosine triphosphate (ATP). The high surface area afforded by the sol-gel derived material allowed high loading of DNA aptamers, while the inclusion of gold nanoparticles within the silica film provided a high refractive index (RI) overlay, which is required to enhance the sensitivity of the LPG sensor according to our numerical simulations. By using a structure-switching aptamer construct that could release an oligonucleotide upon binding of ATP, the effective change in RI was both enhanced and inverted (i.e., binding of ATP caused a net reduction in molecular weight and refractive index), resulting in a system that prevented signals originating from nonspecific binding. This is the first report on the coupling of aptamers to LPG fiber sensors and the first use of high RI AuNP/silica films as supports to immobilize biomolecules onto the LPG sensor surface. The dual functionality of such films to both improve binding density and LPG sensor cladding refractive index results in a substantial enhancement in the sensitivity of such sensors for small molecule detection.

Surface plasmon resonance in monitoring of complement activation on biomaterials

When artificial materials come into contact with blood, various biological responses are induced. For successful development of biomaterials used in biomedical devices that will be exposed to blood, understanding and control of these interactions are essential. Surface plasmon resonance (SPR) spectroscopy is one of the surface-sensitive optical methods to monitor biological interactions. SPR enables real-time and in situ analysis of interfacial events associated with biomaterials research. In this review, we describe an SPR biosensor and its application to monitor complement activation onto biomaterials surface. We also discuss the effect of surface properties of the material on complement activation.

Label-free quantification of cystatin C as an improved marker for renal failure

A label-free biosensor has been developed, allowing quantification of cystatin C in human serum. This was achieved by using reflectometric interference spectroscopy as detection method. Cystatin C is a small serum protein that allows detection of renal failure more reliably than established parameters as creatinine. The protein was immobilized on the surface of a glass transducer, forming the sensitive layer of the sensor chip. Based on a binding-inhibition assay, two different types of monoclonal cystatin C antibodies were compared, by their behavior and their obtained working range in buffer and serum as matrix. Both antibodies allowed quantification of the protein in serum as matrix within the required clinical ranges of 0.53-1.02 mg/L. Detected recovery rates are in a range between 84.8% and 116.1%. The developed sensor shows high inner chip reproducibility and low cross-sensitivity.

Biosensors for diagnostic applications

Biosensors combine a transducer with a biorecognition element and thus are able to transform a biochemical event on the transducer surface directly into a measurable signal. By this they have the potential to provide rapid, real-time, and accurate results in a comparatively easy way, which makes them promising analytical devices. Since the first biosensor was introduced in 1962 as an "enzyme electrode" for monitoring glucose in blood, medical applications have been the main driving force for further biosensor development. In this chapter we outline potential biosensor setups, focusing on transduction principles, biorecognition layers, and biosensor test formats, with regard to potential applications. A summary of relevant aspects concerning biosensor integration in efficient analytical setups is included. We describe the latest applications of biosensors in diagnostic applications focusing on detection of molecular biomarkers in real samples. An overview of the current state and future trends of biosensors in this field is given.