Full-field photonic biosensors based on tunable bio-doped sol-gel glasses

Integrated Photonic Nanofences: Combining Subwavelength Waveguides with an Enhanced Evanescent Field for Sensing Applications

Photonic nanofences consisting of high aspect ratio polymeric optical subwavelength waveguides have been developed for their application into photonic sensing devices. They are up to millimeter long arrays of 250 nm wide and 6 μm high ridges produced by an advanced lithography process on a silicon substrate enabling their straightforward integration into complex photonic circuits. Both simulations and experimental results show that the overlap of the evanescent fields propagating from each photonic nanofence allows for the formation of an effective waveguide that confines the overall evanescent field within its limits. This permits a high interaction with the surrounding medium which can be larger than 90% of the total guided light intensity (approximately 20000 times larger than the evanescent field of a standard waveguide with equivalent dimensions). In this work, we not only investigate the photonic properties of these structures but also demonstrate their successful integration into a photonic sensor. An absorbance-based sensor for the determination of lead in water samples is therefore achieved by the combination of the photonic nanofences with an ion-sensitive optical membrane. The experimental results for lead detection in water show a sensitivity of 0.102 AU/decade, and a linear range between 10(-6) M and 10(-2) M Pb(II). A detection limit as low as 7.3 nM has been calculated according to IUPAC for a signal-to-noise ratio of 3.

Fiber-Optic Chemical Sensors and Fiber-Optic Bio-Sensors

This review summarizes principles and current stage of development of fiber-optic chemical sensors (FOCS) and biosensors (FOBS). Fiber optic sensor (FOS) systems use the ability of optical fibers (OF) to guide the light in the spectral range from ultraviolet (UV) (180 nm) up to middle infrared (IR) (10 μm) and modulation of guided light by the parameters of the surrounding environment of the OF core. The introduction of OF in the sensor systems has brought advantages such as measurement in flammable and explosive environments, immunity to electrical noises, miniaturization, geometrical flexibility, measurement of small sample volumes, remote sensing in inaccessible sites or harsh environments and multi-sensing. The review comprises briefly the theory of OF elaborated for sensors, techniques of fabrications and analytical results reached with fiber-optic chemical and biological sensors.

One-step patterning of hybrid xerogel materials for the fabrication of disposable solid-state light emitters

The one-step room-temperature micropatterning of a fluorophore-doped xerogel material on silicon oxide substrates is reported. The organo-alkoxysilane precursors and organic fluorescent dyes, as well as the polymerization experimental conditions, were tailored in order to obtain a highly homogeneous transparent material suitable for photonic applications. A thorough structural characterization was carried out by Fourier transform infrared (FT-IR) spectroscopy, (29)Si nuclear magnetic resonance ((29)Si NMR), thermogravimetric analysis (TGA), N(2) adsorption Brunauer-Emmett-Teller (BET) porosimetry, and confocal microscopy. These studies revealed a stable nonporous highly cross-linked polymer network containing evenly dispersed fluorescent molecules. Xerogel microstructures having thicknesses between 4 and 80 μm and height-to-width ratios between 0.04 and 4, as well as showing different geometries, from well arrays to waveguides, were patterned in a single step by micromolding in capillaries (MIMIC) soft lithographic technique. The reliability of the replication process was tested by bright-field optical microscopy and scanning electron microscopy (SEM) that show the close fidelity of the microstructures to the applied mold. The optical performance of the developed material was demonstrated by fabricating waveguides and evaluating their corresponding spectral response, obtaining absorption bands, at the expected excitation wavelengths of the corresponding fluorescent dyes and gain due to photonic re-emission (fluorescence) at their corresponding dye emission wavelengths. The hybrid xerogel material and the application of the simple fabrication technology presented herein can be directly applied to the development of cost-effective photonic components, as could be light emitters, to be readily integrated in single-use lab-on-chip devices and other polymeric microsystems.

Pneumatically tunable optofluidic 2 × 2 switch for reconfigurable optical circuit

We presented a pneumatically tunable 2 × 2 optofluidic switch for on-chip light routing that was controlled by compressed air. The device was fabricated with an optically clear elastomer-polydimethylsiloxane (PDMS)-by soft-lithography. The optical switching is realized with a tunable air-gap mirror by which the light is deflected due to total internal reflection in the bypass state. When the device is subjected to high pressure, the air gap collapses and hence the light will be switched to the crossover state. The device had a switching speed of more than 5 Hz and an extinction ratio of 8 dB. This switch can be readily integrated with other microfluidic circuits. We demonstrated a simple reconfigurable optical waveguide circuit for dual-channel microfluidic spectroscopy measurement on a chip.

A single-layer, planar, optofluidic Mach-Zehnder interferometer for label-free detection

We have developed a planar, optofluidic Mach-Zehnder interferometer for the label-free detection of liquid samples. In contrast to most on-chip interferometers which require complex fabrication, our design was realized via a simple, single-layer soft lithography fabrication process. In addition, a single-wavelength laser source and a silicon photodetector were the only optical equipment used for data collection. The device was calibrated using published data for the refractive index of calcium chloride (CaCl(2)) in solution, and the biosensing capabilities of the device were tested by detecting bovine serum albumin (BSA). Our design enables a refractometer with a low limit of detection (1.24 × 10(-4) refractive index units (RIU)), low variability (1 × 10(-4) RIU), and high sensitivity (927.88 oscillations per RIU). This performance is comparable to state-of-the-art optofluidic refractometers that involve complex fabrication processes and/or expensive, bulky optics. The advantages of our device (i.e. simple fabrication process, straightforward optical equipment, low cost, and high detection sensitivity) make it a promising candidate for future mass-producible, inexpensive, highly sensitive, label-free optical detection systems.

Fluorophore-doped xerogel antiresonant reflecting optical waveguides

Rhodamine B and Alexa Fluor 430 fluorophores have been used as doping agents for xerogel waveguides defined over an antiresonant (ARROW) filter. This configuration has a significant level of integration, since it merges the waveguide, the light emitter and the filter in a single photonic element. Different technologies have been combined for their implementation, namely soft lithography, standard silicon-based technology and silicon bulk micromachining. The spectral response of 15-mm long waveguides without fluorophore is first analyzed as a function of the waveguide width. Here, it has been observed how the xerogel used has a high transparency in the visible spectra, having only significant absorption at the wavelength where the ARROW filter is in resonance. In a second step, identical waveguides but doped with two different concentrations of Rhodamine B and Alexa Fluor 430 are studied. In addition to the effect of the filter, fluorophore-doped xerogel waveguides show losses close to -2 dB (equivalent to 2 dB of light emission). In addition, it has been observed how an increase of the fluorophore concentration within the xerogel matrix does not provide with a emission increase, but saturation or even a decrease of this magnitude due to self-absorption. Finally, the total losses of the proposed waveguides are analyzed as a function of their width, obtaining losses close to 5 dB for waveguide widths higher than 50 µm.

Poly(dimethylsiloxane) photonic microbioreactors based on segmented waveguides for local absorbance measurement

We present the development of microbioreactors (MBRs) based on poly(dimethylsiloxane) (PDMS) segmented waveguides (SWG) for local absorbance measurements. Two different MBRs were studied, either using symmetric or asymmetric SWG (being defined as MBR-S and MBR-A, respectively). Their optical and fluidic performances were numerically analyzed, showing robustness from an optical point of view and distinct fluid flow profile. The optical characterization was done in two steps. Initially, the experimental limit of detection (LOD) and the sensitivity were determined for two different analytes (fluorescein and methylorange). With both systems, a similar limit of detection for both analytes was obtained, being in the micromolar level. Their sensitivities were 20.2±0.3 (×10⁻³) A.U./μM and 5.5±0.2 (×10⁻³) A.U./μM for fluorescein and methylorange, respectively. Once validated its applicability for local absorbance measurements, a continuous cultivation of Saccharomyces cerevisiae was done to test the viability of the proposed systems for photonic MBRs. Concretely, the cell growth was locally monitored inside the MBR during 33 h. Spectral analysis showed that the determination of the culture parameters were wavelength dependant, with a growth rate of 0.39±0.02 h⁻¹ and a doubling time of 1.65±0.09 h at an optimal wavelength of 469.9±0.3 nm. Besides the easy and monolithic integration of the SWG into poly(dimethylsiloxane) microfluidic systems, the results presented here are very promising for the application in any disposable photonic lab-on-a-chip systems used for online analysis or photonic MBRs.

Tunable two-dimensional liquid gradient refractive index (L-GRIN) lens for variable light focusing

We report a two-dimensional (2D) tunable liquid gradient refractive index (L-GRIN) lens for variable focusing of light in the out-of-plane direction. This lens focuses a light beam through a liquid medium with a 2D hyperbolic secant (HS) refractive index gradient. The refractive index gradient is established in a microfluidic chamber through the diffusion between two fluids with different refractive indices, i.e. CaCl(2) solution and deionized (DI) water. The 2D HS refractive index profile and subsequently the focal length of the L-GRIN lens can be tuned by changing the ratio of the flow rates of the CaCl(2) solution and DI water. The focusing effect is experimentally characterized through side-view and top-view image analysis, and the experimental data match well with the results from ray-tracing optical simulations. Advantages of the 2D L-GRIN lens include simple device fabrication procedure, low fluid consumption rate, convenient lens-tuning mechanism, and compatibility with existing microfluidic devices. We expect that with further optimizations, this 2D L-GRIN lens can be used in many optics-based lab-on-a-chip applications.

State-of-the-art of (bio)chemical sensor developments in analytical Spanish groups

(Bio)chemical sensors are one of the most exciting fields in analytical chemistry today. The development of these analytical devices simplifies and miniaturizes the whole analytical process. Although the initial expectation of the massive incorporation of sensors in routine analytical work has been truncated to some extent, in many other cases analytical methods based on sensor technology have solved important analytical problems. Many research groups are working in this field world-wide, reporting interesting results so far. Modestly, Spanish researchers have contributed to these recent developments. In this review, we summarize the more representative achievements carried out for these groups. They cover a wide variety of sensors, including optical, electrochemical, piezoelectric or electro-mechanical devices, used for laboratory or field analyses. The capabilities to be used in different applied areas are also critically discussed.

Advances in biomimetic and nanostructured biohybrid materials

The rapid increase of interest in the field of biohybrid and biomimetic materials that exhibit improved structural and functional properties is attracting more and more researchers from life science, materials science, and nanoscience. Concomitant results offer valuable opportunities for applications that involve disciplines dealing with engineering, biotechnology, medicine and pharmacy, agriculture, nanotechnology, and others. In the current contribution we collect recent illustrative examples of assemblies between materials of biological origin and inorganic solids of different characteristics (texture, structure, and particle size). We introduce here a general overview on strategies for the preparation and conformation of biohybrids, the synergistic effects that determine the final properties of these materials, and their diverse applications, which cover areas as different as tissue engineering, drug delivery systems, biosensing devices, biocatalysis, green nanocomposites, etc.

Polymeric acrylate-based hybrid films containing lead and iron patterned by UV photo-polymerization

The development and processing of hybrid inorganic-organic thin film materials plays a critical role in advancing interdisciplinary sciences and device manufacturing. Here we present a novel approach to synthesize and deposit acrylate-containing organic/inorganic hybrid films. The material is based on a chemical solution and includes specifically desired metal dopants that are fully integrated into the backbone of the polymer structure. The film can be deposited by simple spin coating, and we confer photosensitive properties to the material making it directly patterned by traditional UV photolithography techniques. Film thickness, chemical characterization, and wet/dry etching capability of the film are also investigated. We believe this innovative material has the potential to be used in a broad range of applications for electronic, photonic, biology, and other interdisciplinary fields.