Gold and silver nanoparticle-based colorimetric sensors: New trends and applications

Gold and Silver nanoparticles (AuNPs and AgNPs) are perfect platforms for developing sensing colorimetric devices thanks to their high surface to volume ratio and distinctive optical properties, particularly sensitive to changes in the surrounding environment. These characteristics ensure high sensitivity in colorimetric devices. Au and Ag nanoparticles can be capped with suitable molecules that can act as specific analyte receptors, so highly selective sensors can be obtained. This review aims to highlight the principal strategies developed during the last decade concerning the preparation of Au and Ag nanoparticle-based colorimetric sensors, with particular attention to environmental and health monitoring applications

Tapered Optical Microfibre Based Structures for Sensing Applications

There has been an increasing demand in recent years from a wide variety of industries for sensors which combine high sensitivity, fast response, compact size and low power consumption. Tapered optical microfibres can generate easily accessible evanescent fields with a large intensity and short decay distance which make microfibres very suitable candidates as the basis of sensors to suit a variety of application areas. In this thesis, experimental research is presented concerning the development of sensors using structures based on tapered optical microfibres, with a particular emphasis on biochemical sensing applications. Light propagation along an optical microfibre depends on its shape, diameter and surface roughness. A microfibre fabrication setup developed as a key prerequisite to the research undertaken, that utilized an adapted microheater brushing and tapering technique is described. The setup allows for the fabrication of microfibres and related structures with controllable taper shapes and diameters. There is a tradeoff between sensitivities and microfibre diameters (which directly affects the robustness of the microfibre structures) for microfibre based sensors. To mitigate this tradeoff, two microfibre based structures were chosen and investigated for sensor development in the research reported in this thesis. The first structure was an optical microfibre coupler. Such an optical microfibre coupler, which has environment dependent coupling coefficients in addition to easily accessible evanescent fields, is a simple and efficient structure for sensing. A refractive index sensor with a maximum sensitivity of 4155 nm/RIU was developed using an optical microfibre coupler. Utilizing the structure’s refractive index sensitivity, a humidity sensor was developed by coating a microfibre coupler with a layer of humidity sensitive polymer. A biosensor was also developed by immobilizing a bio-receptor on the surface of a packaged microfibre coupler. The ability of the developed biosensor to detect the specific binding between an antibody-antigen pairing for potential applications in clinical diagnostics was demonstrated and is reported in this thesis. The second structure was a tapered optical microfibre which incorporates gold-silver alloy nanoparticles. By immobilizing nanoparticles onto the surface of a tapered optical. microfibre to generate localized surface plasmon resonances, sensitivity enhancement can be achieved for microfibres with relatively large diameters, which has the benefit of being more mechanically robust. The use of gold-silver alloy nanoparticles with different alloy formulations can offer the extra advantage of tunable physicochemical properties. The localized surface plasmon resonance effects were investigated and compared for sensor samples incorporating nanoparticles with different alloy formulations. As an example of a sensing application using the structure, a novel pH sensor was demonstrated by coating the immobilized nanoparticles with a pH sensitive polyelectrolyte multilayer film.

Plasmonic microstructured optical fibres: an efficient platform towards biosensing

Detecting life-threatening diseases is a major challenge in biomedicine, as it requires pathogen identication on the molecular level. One promising detection strategy relies on attaching molecular probes to nanoparticles (NPs), which support localised surface plasmon resonance (LSPR). Probe-functionalised NPs can then detect molecular DNA-binding events via a macroscopic change in their optical response. Until now, NP-sensing schemes have been primarily implemented using planar substrates, requiring complex launching techniques and cost-intensive microscopy. An alternative approach adopted in this work involves inltrating optical bres with NPs allowing LSPR excitation and spectral multiplexing within one device. The principle idea relies on probing deposited plasmonic NPs by propagating optical fibre modes, leading to hybrid plasmonic-photonic fibres for biosensing. An important class of innovative bres exploited in this work are microstructured optical bres (MOFs) containing longitudinally invariant microstructures. These structures enable unprecedented adjustment of light matter interaction resulting in a high degree of sensitivity and an optofluidic environment ideally suited for biochemical application. In this thesis optofluidic channels are integrated in direct proximity to the light guiding core, boosting the light-analyte interaction length by orders of magnitude. This concept thus represents a multiscale approach, fundamentally connecting the microscopic level via LSPR-mediated sensing with the macroscopic world using MOFs leading to a novel and unexplored sensor platform. This study shows that combining plasmonic-bre waveguides with microuidics yields a highly integrated, reusable, optouidic interface for efcient refractive index sensing with outlook for DNA diagnostics. This unique combination is extremely attractive from both device and clinical point of view, as the flexible handling of optical fibres principally enables in-vivo application

Emerging (Bio)Sensing Technology for Assessing and Monitoring Freshwater Contamination - Methods and Applications

Ecological Water Quality - Water Treatment and ReuseWater is life and its preservation is not only a moral obligation but also a legal requirement. By 2030, global demands will exceed more than 40 % the existing resources and more than a third of the world's population will have to deal with water shortages (European Environmental Agency [EEA], 2010). Climate change effects on water resources will not help. Efforts are being made throughout Europe towards a reduced and efficient water use and prevention of any further deterioration of the quality of water (Eurostat, European Comission [EC], 2010). The Water Framework Directive (EC, 2000) lays down provisions for monitoring, assessing and classifying water quality. Supporting this, the Drinking Water sets standards for 48 microbiological and chemical parameters that must be monitored and tested regularly (EC, 1998). The Bathing Water Directive also sets concentration limits for microbiological pollutants in inland and coastal bathing waters (EC, 2006), addressing risks from algae and cyanobacteria contamination and faecal contamination, requiring immediate action, including the provision of information to the public, to prevent exposure. With these directives, among others, the European Union [EU] expects to offer its citizens, by 2015, fresh and coastal waters of good quality

Recent advances in optical fiber devices for microfluidics integration

This paper examines the recent emergence of miniaturized optical fiber based sensing and actuating devices that have been successfully integrated into fluidic microchannels that are part of microfluidic and lab-on-chip systems. Fluidic microsystems possess the advantages of reduced sample volumes, faster and more sensitive biological assays, multi-sample and parallel analysis, and are seen as the de facto bioanalytical platform of the future. This paper considers the cases where the optical fiber is not merely used as a simple light guide delivering light across a microchannel, but where the fiber itself is engineered to create a new sensor or tool for use within the environment of the fluidic microchannel

Optical Biosensors for Label-Free Detection of Small Molecules

Label-free optical biosensors are an intriguing option for the analyses of many analytes, as they offer several advantages such as high sensitivity, direct and real-time measurement in addition to multiplexing capabilities. However, development of label-free optical biosensors for small molecules can be challenging as most of them are not naturally chromogenic or fluorescent, and in some cases, the sensor response is related to the size of the analyte. To overcome some of the limitations associated with the analysis of biologically, pharmacologically, or environmentally relevant compounds of low molecular weight, recent advances in the field have improved the detection of these analytes using outstanding methodology, instrumentation, recognition elements, or immobilization strategies. In this review, we aim to introduce some of the latest developments in the field of label-free optical biosensors with the focus on applications with novel innovations to overcome the challenges related to small molecule detection. Optical label-free methods with different transduction schemes, including evanescent wave and optical fiber sensors, surface plasmon resonance, surface-enhanced Raman spectroscopy, and interferometry, using various biorecognition elements, such as antibodies, aptamers, enzymes, and bioinspired molecularly imprinted polymers, are reviewed

Aptamers Targeting Membrane Proteins for Sensor and Diagnostic Applications.

Many biological processes (physiological or pathological) are relevant to membrane proteins (MPs), which account for almost 30% of the total of human proteins. As such, MPs can serve as predictive molecular biomarkers for disease diagnosis and prognosis. Indeed, cell surface MPs are an important class of attractive targets of the currently prescribed therapeutic drugs and diagnostic molecules used in disease detection. The oligonucleotides known as aptamers can be selected against a particular target with high affinity and selectivity by iterative rounds of in vitro library evolution, known as Systematic Evolution of Ligands by EXponential Enrichment (SELEX). As an alternative to antibodies, aptamers offer unique features like thermal stability, low-cost, reuse, ease of chemical modification, and compatibility with various detection techniques. Particularly, immobilized-aptamer sensing platforms have been under investigation for diagnostics and have demonstrated significant value compared to other analytical techniques. These "aptasensors" can be classified into several types based on their working principle, which are commonly electrochemical, optical, or mass-sensitive. In this review, we review the studies on aptamer-based MP-sensing technologies for diagnostic applications and have included new methodological variations undertaken in recent years

Surface Plasmon Resonance for Biosensing

The rise of photonics technologies has driven an extremely fast evolution in biosensing applications. Such rapid progress has created a gap of understanding and insight capability in the general public about advanced sensing systems that have been made progressively available by these new technologies. Thus, there is currently a clear need for moving the meaning of some keywords, such as plasmonic, into the daily vocabulary of a general audience with a reasonable degree of education. The selection of the scientific works reported in this book is carefully balanced between reviews and research papers and has the purpose of presenting a set of applications and case studies sufficiently broad enough to enlighten the reader attention toward the great potential of plasmonic biosensing and the great impact that can be expected in the near future for supporting disease screening and stratification

Development of LSPR-based optical biosensors for the label-free detection of biomolecular interactions in high-density peptide arrays

Peptide or protein chips which can track hundreds or thousands of distinct proteins from a blood or urine sample at a single step are highly desired. It will allow the diagnosis of diseases in their formative, treatable stages just by detecting proteins that are markers for these specific diseases. To simultaneously and efficiently detect where blood´s proteins bind on the grid, label-free detection methods are favorable in order to reduce time and cost demands and facilitate the detection of low-affinity binding events. In this work, a label-free biosensor based on localized surface plasmon resonance (LSPR) was developed and specifically optimized regarding its application as a solid substrate in the synthesis of high-density peptide arrays, and in the detection of molecular interactions occurring on the sensor surface. Three main issues which are crucial in achieving this goal have been covered in this work: (i) development and optimization of the LSPR biosensor, (ii) Synthesis of a protein resistant layer on the LSPR biosensor, (iii) Label-free detection of biomolecular interactions on the polymer coated LSPR biosensors. For this purpose, a LSPR-based biosensor was first constructed. It consists of two gold layers and an intermediate dielectric layer in-between. The LSPR biosensor shows several pronounced resonance peaks in the UV-visible region of the electromagnetic spectrum, which are highly sensitive to changes in the refractive index of the surrounding medium. The LSPR biosensor was optimized in terms of plasmon resonance line shape, optical homogeneity, and sensitivity to facilitate its application in the label-free detection of biomolecular interactions in array format. In a second step, a poly(ethylene glycol) based polymer was synthesized on the sensor surface at mild reaction conditions by using Atom Transfer Radical Polymerization (ATRP). The sensor was first coated with a silica gel to stabilize the sensor and provide a sufficiently high number of functional groups. ATRP initiators were immobilized on the silica gel surface in 2 steps by silanization and esterification to enhance the coupling efficiency. Subsequently, polymerization was carried out with oligo(ethylene glycol) methacrylate (OEGMA) as the monomer resulting in a poly(ethylene glycol) methacrylate (PEGMA) polymer film. In the second approach, a graft copolymer film was synthesized on the sensor surface with methyl methacrylate (MMA) as the diluting monomer in order to reduce the protein-resistance of the sensor coating and further enhance the sensor stability. Graft copolymer films with 10% PEGMA/ 90% MMA and a thickness of 50-60 nm were successfully synthesized by setting the polymerization time between 9.5 and 10.5 hours. This thickness regime is required to ensure a high-loading of amino functional groups, which serve as the starting point for the subsequent peptide array synthesis. At the same time, this film thickness does not exceed the surface sensitivity regime of the LSPR biosensor. To test the performance of the LSPR biosensor, an array of fluorescent-labeled antibodies was formed on its surface by a spotting robot. The LSPR image displays an array of spots which corresponds to the fluorescence image. Moreover, the quantity of antibody bound to the sensor surface was correctly predicted based on the measured wavelength shifts in the quadrupole regime and a mass sensitivity factor known from literature. This shows that the LSPR biosensor described in this thesis has the potential to allow the detection of molecular interactions in a miniaturized array format in a quantitative manner. In the final part of this thesis, the polymer-coated LSPR biosensor was successfully utilized as the solid substrate in the synthesis of a peptide array via a novel laser printing technique developed at the Cancer Research Center Heidelberg (DKFZ). An array with 9x20 variants of hemagglutinin/HA (YPYDVPDYA) epitope was synthesized on a polymer-coated LSPR biosensor and incubated with IR-dye conjugated specific antibody. The LSPR image was generated by evaluating the wavelength shift in the octapole resonance peak and has successfully displayed the entire peptide array formed on the sensor surface. In a separate study, the potential of single, small particles for biosensing applications in miniaturized format was investigated. Whispering gallery modes (WGM) of fluorescence-doped sulfonated polystyrene (PS) particles with a diameter of 2 µm were studied with respect to their resonance shift after adsorption of polyelectrolyte multilayers. The resonance shifts were plotted as a function of the film thickness and a slope of 0.038 nm/ Å was obtained from a linear fit to the experimental data. This sensitivity factor can be translated into a detection limit of 3 fg by assuming a spectral resolution of 0.1 nm and a polyelectrolyte mass density17 of 0.81 g/cm3