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This review provides a historical perspective along with a tutorial on the concepts, instrumentation, and progress in the chemical and biological sensing applications of leaky waveguides (LWs) analytical use. The review compares the sensing performance (i.e. refractive index sensitivity, figure of merit, refractive index resolution, and absorption/ scattering/ fluorescence sensitivity) of LWs with other label-free optical sensors. As the porosity of the waveguide layer is a primary factor in determining the sensing performance of the LWs, different materials and fabrication methods used to make porous films are also reviewed. Finally, we have discussed the challenges that need to be addressed and the opportunities that can be availed for the development and adoption of LWs for chemical and biological sensing at point-of-need (PoN).
This report describes a Young interferometer (YI) sensor for real-time imaging of refractive index (RI) along the length of microchannels. The YI comprised a unique combination of Fresnel biprism and cylindrical lens to obtain two wedge shaped light beams, which ultimately overlapped to produce the interference fringes. The measured temporal and spatial resolution of the reported YI is 2 s and 295 µm respectively. The RI resolution per mm of the optical pathlength of the YI sensor is 2.04 × 10−6 while providing information on spatial RI distribution in real-time. The ability of the reported YI to image RI in real-time was used to monitor electrokinetic transport of a model protein, bovine serum albumin (BSA), in agarose filled channels of a microfluidic device. This method is particularly suitable to planar µTAS devices as it is free-space, senses through the entire depth of the channel and requires no coupling devices such as gratings or prisms.
Front Cover: In article number 1900228 by Ruchi Gupta and co‐workers, a water‐soluble chemical method for making a photo‐functionalizable hydrogel is reported for the fabrication of optical grating sensors with high (>3) aspect ratio. This grating sensor contains a dye that is sensitive to pH, resulting in changes in the diffraction pattern as the pH of the solution in which the grating is immersed changes.
Improving the limit of detection by preconcentration and reducing the response time of optical biosensors are key requirements to enable their use in point of care (PoC) applications. To address these requirements, we have shown that integration of isoelectric focusing (IEF) at a pH step with a leaky waveguide (LW) sensor containing a non-specific affinity ligand (reactive blue 4 dye (RB4)) can reduce the limit of detection of an exemplar protein (bovine serum albumin (BSA)) by a factor of 600–930 and reduce the response time to < 60 s. This is 6–9 times better preconcentration and up to 16 times faster response time than previous reports. IEF was performed with standard ampholytes and with simple acids and bases forming a pH step. Using ampholytes gave good preconcentration, but was much slower than using a pH step. The LW sensor used a thin agarose hydrogel layer into which RB4 was immobilized. The dye acted both as a non-specific affinity ligand and to visualize the waveguide resonances. This allowed the refractive index of the waveguide to be monitored in real time at any point along the 10 mm separation channel length.
Volume amplitude gratings made of mesoporous hydrogels are beneficial for sensing, but are difficult to fabricate because they involve creating high aspect ratio features in soft materials. A novel photofunctionalizable hydrogel is reported and its suitability for fabricating grating sensors is demonstrated comprising of features with an aspect ratio of ≈3.2. To make a photofunctionalizable hydrogel with high optical quality that can be patterned using widely available light sources, a water‐soluble photoactive monomer and sensitizer are synthesized. A transmission amplitude grating is subsequently fabricated in a ≈100 µm thick photofunctionalizable hydrogel film by reaction of the free amines generated in the photoexposed regions with pH‐responsive fluorescein isothiocyanate. The volume hydrogel grating described herein is shown to be suitable for real‐time sensing of pH as an exemplar analyte. This work will have a significant impact on the fabrication of diffractive optical structures in thick films of hydrogels that are highly promising for biomolecular sensing in disease diagnosis and healthcare monitoring. A novel photofunctionalizable hydrogel is used for the fabrication of diffractive optical sensors by spatial patterning of analyte‐sensitive moieties in the entire depth of the material. This diffractive sensor contains a dye that is sensitive to pH, resulting in changes in the diffraction pattern, as the pH of the solution in which the grating is immersed changes.
We report a method where the refractive index increments of an iron storage protein, ferritin, and apoferritin (ferritin minus iron) were measured over the wavelength range of 450 to 678 nm to determine the average iron content of the protein. The protein used in this study had ~3375 iron atoms per molecule. The measurement of optical dispersion over the broad wavelength range was enabled by the use of mesoporous leaky waveguides made of chitosan. We present a facile approach for fabricating mesoporous chitosan waveguides for improving the measurement sensitivity of macromolecules such as ferritin. Mesoporous materials allow macromolecules to diffuse into the waveguide, maximizing their interaction with the optical mode and thus increasing sensitivity by a factor of ~9 compared to non-porous waveguides. The sensitivity was further improved and selectivity towards ferritin was achieved by the incorporation of antibodies in the waveguide. The method presented in this work is a significant advance over state-of-art method, enzyme linked immunosorbent assay (ELISA) used in clinics, because it allows determining the average content of ferritin in a single step. The average iron content of ferritin is an important marker for conditions such as injury, inflammation and infection, Thus, the here presented approach of measuring optical dispersion to determine the average iron content of ferritin has a significant potential to improve the point-of-care analysis of the protein for disease diagnosis and screening.
Optical structures made of photo patternable hydrogels have emerged as promising multi-analyte biosensors  for point-of-care diagnostics and chronic wound monitoring such as diabetic foot ulcers (DFUs), which pose significant societal challenges and currently cost ~£3 billion p.a. in the UK alone [2, 3]. Additionally, the incidence of chronic wounds is predicted to increase as a result of lifestyle changes and an ageing population. To fabricate these optical structures with spatial and temporal control of immobilisation of biorecognition species, we developed a photo patternable polyacrylamide-aminopropylmethacrylate based hydrogel where the amine functionalities (-NH2) were caged with a photolabile group, 4,5-Dimethoxy-2-nitrobenzyl chloroformate (NVOC). The water soluble monomers bearing the photo caged group and the resulting hydrogels were well characterised by various analytical techniques. The density of uncaging of amine functionality (-NH2) was studied as a function of exposure time using UV/Vis (λ = 365 nm/405 nm) light and the spatial resolution of resulting functional patterns was studied by fluorescence imaging after the attachment of fluorescein isothiocyanate (FITC) to the uncaged amines. Our studies also confirm that density of uncaged amines is proportional to the exposure time. With our photo-responsive material we controlled the immobilisation of biorecognition species with the functional groups generated by successive light exposures for the development of multi-analyte biosensors (Figure 1).
Simultaneous refractive index monitoring and absorption spectroscopy in small volumes common to both measurements using single light source and detector are beneficial, but challenging to perform. This work presents an optical device consisting of a porous waveguide deposited on glass, which is called dye-doped leaky waveguide (DDLW), for this purpose. The waveguide was made of agarose doped with a dye, reactive blue 4. Glycerol and rhodamine 6G were used as an exemplar system to demonstrate the feasibility of DDLW to perform simultaneous refractive index and broadband absorption measurements in a small common volume of 3.5 nL. In this case, the immobilised reactive blue 4 permits visualisation of the resonance angle of the waveguide for refractive index monitoring, while the additional absorption caused by the free rhodamine 6G was determined by measuring the wavelength dependent additional losses in the reflectivity profile of the DDLW. The refractive index sensitivity and limit of detection of the DDLW was 106.32 ± 0.97° RIU−1 and 2.82 × 10−6 respectively. By combining the two measurements, the DDLW was shown to be suitable to monitor the absorption spectrum and anomalous dispersion of rhodamine 6G as well as the nature of its interactions with reactive blue 4. These interactions resulted in enhancing the concentration of the rhodamine 6G in the waveguide by a factor between 119 and 191. The DDLW was shown to be suitable to obtain the absorption spectrum of rhodamine 6G at concentrations as low as 1 μM. Future work will focus on the application of the DDLW to characterise analytes/determine parameters of biological significance such as iron loading per molecule of serum ferritin.
We report a polyacrylamide-allylamine based hydrogel where the amine groups are caged with a photolabile group, 4,5-Dimethoxy-2-nitrobenzyl chloroformate (NVOC) to fabricate optical structures.