Reversible colorimetric sensing of volatile analytes by wicking in close proximity to a photonic film

Isolation of volatile analytes from environmental or biological fluids is a rate-determining step that can delay the response time for continuous sensing. In this paper, we demonstrate a colorimetric sensing system that enables the rapid detection of gas-phase analytes released from a flowing micro-volume fluid sample. The sensor platform is an analyte-responsive metal-insulator-metal (MIM) thin-film structure integrated with a large area quartz micropillar array. This allows precise planar alignment and microscale separation (310 μm) of the optical and fluidic structures. This configuration offers rapid and homogeneous color changes over large areas that permits detection by low-resolution optics or eye, which is well-suited to portable/wearable devices. For our proof-of-principle demonstration, we utilized a poly(methyl methacrylate) (PMMA) spacer and evaluated the sensor's response (color change) to ethanol vapor. We show that the RGB color value is quantitatively linked to the spacer swelling, which is reversible and repeatable. The optofluidic platform reduces the sensor response time from minutes to seconds compared with experiments using a conventional chamber. The sensor's concentration-dependent response was examined, confirming the potential of the reported sensing platform for continuous, compact, and quantitative colorimetric analysis of volatile analytes in low-volume samples, such as biofluids.

Nanoporous thin films in optical waveguide spectroscopy for chemical analytics

Spectroscopy with planar optical waveguides is still an active field of research for the quantitative analysis of various supramolecular surface architectures and processes, and for applications in integrated optical chip communication, direct chemical sensing, etc. In this contribution, we summarize some recent development in optical waveguide spectroscopy using nanoporous thin films as the planar substrates that can guide the light just as well as bulk thin films. This is because the nanoporosity is at a spacial length-scale that is far below the wavelength of the guided light; hence, it does not lead to an enhanced scattering or additional losses of the optical guided modes. The pores have mainly two effects: they generate an enormous inner surface (up to a factor of 100 higher than the mere geometric dimensions of the planar substrate) and they allow for the exchange of material and charges between the two sides of the solid thin film. We demonstrate this for several different scenarios including anodized aluminum oxide layers for the ultrasensitive determination of the refractive index of fluids, or the label-free detection of small analytes binding from the pore inner volume to receptors immobilized on the pore surface. Using a thin film of Ti metal for the anodization results in a nanotube array offering an even further enhanced inner surface and the possibility to apply electrical potentials via the resulting TiO₂ semiconducting waveguide structure. Nanoporous substrates fabricated from SiN_x thin films by colloid lithography, or made from SiO₂ by e-beam lithography, will be presented as examples where the porosity is used to allow for the passage of ions in the case of tethered lipid bilayer membranes fused on top of the light-guiding layer, or the transport of protons through membranes used in fuel cell applications. The final example that we present concerns the replication of the nanopore structure by polymers in a process that leads to a nanorod array that is equally well suited to guide the light as the mold; however, it opens a totally new field for integrated optics formats for direct chemical and biomedical sensing with an extension to even molecularly imprinted structures. Graphical abstract.

Free-standing hydrogel-particle composite membrane with dynamically controlled permeability

The preparation and investigation of a free-standing membrane made from a composite of thermoresponsive poly(N-isopropylacrylamide) (pNIPAAm) and polystyrene nanoparticles (PS NP) with temperature-controlled permeability is reported. The method exploits the light-induced crosslinking of the photo-reactive pNIPAAm-based polymer and mechanical reinforcement of the membrane structure by the polystyrene nanoparticles. About micrometer thick layers were either directly attached to a gold surface or prepared as free-standing layers spanning over arrays of microfluidic channels with a width of about hundred microns by using template stripping. Diffusion of liquid medium, low molecular weight molecules, and large molecular weight proteins contained in blood through the composite membrane was observed with combined surface plasmon resonance (SPR) and optical waveguide spectroscopy (OWS). The swelling ratio, permeability, and nonspecific sorption to these composite membranes were investigated by SPR and OWS as a function of molecular weight of analyte, loading of PS NP in the composite film, and temperature. The authors show successful preparation of a defect-free membrane structure that acts as a thermoresponsive filter with nanoscale pores spanning over an area of several square millimeters. This membrane can be reversibly switched to block or allow the diffusion of low mass molecules to the sensor surface by temperature-triggered swelling and collapsing of the hydrogel component. Blocking of diffusion and low unspecific sorption of proteins contained in blood serum is observed. These features make this platform interesting for potential future applications in continuous monitoring biosensors for the analysis of low molecular weight drug analytes or for advanced cell-on-chip microfluidic studies.