3D Printing for Low-Cost and Versatile Attenuated Total Reflection Infrared Spectroscopy

4D-printed needle panel meters coupled with enzymatic derivatization for reading urea and glucose concentrations in biological samples

On-site analytical techniques continue being developed with advances in modern technology. To demonstrate the applicability of four-dimensional printing (4DP) technologies in the direct fabrication of stimuli-responsive analytical devices for on-site determination of urea and glucose, we used digital light processing three-dimensional printing (3DP) and 2-carboxyethyl acrylate (CEA)-incorporated photocurable resins to fabricate all-in-one needle panel meters. When adding a sample having a value of pH above the pKa of CEA (ca. 4.6-5.0) into the fabricated needle panel meter, the [H+]-responsive layer of the needle, printed using the CEA-incorporated photocurable resins, swelled as a result of electrostatic repulsion among the dissociated carboxyl groups of the copolymer, leading to [H+]-dependent bending of the needle. When coupled with a derivatization reaction (urease-mediated hydrolysis of urea to decrease [H+]; glucose oxidase-mediated oxidization of glucose to increase [H+]), the bending of the needle allowed reliable quantification of urea or glucose when referencing pre-calibrated concentration scales. After method optimization, the method's detection limits for urea and glucose were 4.9 and 7.0 μM, respectively, within a working concentration range from 0.1 to 10 mM. We verified the reliability of this analytical method by determining the concentrations of urea and glucose in samples of human urine, fetal bovine serum, and rat plasma with spike analyses and comparing the results with those obtained using commercial assay kits. Our results confirm that 4DP technologies can allow the direct fabrication of stimuli-responsive devices for quantitative chemical analysis, and that they can advance the development and applicability of 3DP-enabling analytical methods.

Spectroelectrochemical sensing of reaction intermediates and products in an affordable fully 3D printed device

Recent advances in fused deposition modelling 3D printing (FDM 3DP) and synthesis of printable electrically conductive materials enabled the manufacture of customized electrodes and electrochemical devices by this technique. The past couple of years have seen a boom in applying approaches of FDM 3DP in the realm of spectroelectrochemistry (SEC). Despite significant progress, reported designs of SEC devices still rely on conventionally manufactured optical components such as quartz windows and cuvettes. To bridge this technological gap, in this work we apply bi-material FDM 3DP combining electrically conductive and optically translucent filaments to manufacture working electrodes and cells, constituting a fully integrated microfluidic platform for transmission absorption UV-Vis SEC measurements. The cell design enables de-aeration of samples and their convenient handling and analysis. Employing cyclic voltammetric measurements with ruthenium(III) acetylacetonate, ethylviologen dibromide and ferrocenemethanol redox-active probes as model analytes, we demonstrate that the presented platform allows SEC sensing of reactants, intermediates and products of charge transfer reactions, including the inspection of their long-term stability. Approaches developed and presented in this work pave the way for manufacturing customized SEC devices with dramatically reduced costs compared to currently available commercial platforms.

Infrared spectroscopic monitoring of solid-state processes

We put a spotlight on IR spectroscopic investigations in materials science by providing a critical insight into the state of the art, covering both fundamental aspects, examples of its utilisation, and current challenges and perspectives focusing on the solid state.

ATR-Spin: an open-source 3D printed device for direct cytocentrifugation onto attenuated total reflectance crystals

Infrared spectroscopy (IR) enables the direct and rapid characterization of cells at the molecular level. Achieving a rapid and consistent cell preparation is critical for the development of point-of-care diagnostics for cell analysis. Here we introduce an open-source, 3D printed device for integrating the isolation, preconcentration, and measurement of attenuated total reflectance IR spectra of cells from biofluids. The tool comprises a disposable card for cytocentrifugation, equipped with magnets, which allows reproducible integration into the pathlength of the IR spectrophotometer. Preliminary results using cell culture media containing A549 cells indicate that this system enables a qualitative and quantitative characterization of cells down to 10 cells μL^-1 by using a single and cost-effective device and within a few minutes.

Reconsidering the Need for Empirical Alignment and Wavelength Calibration Steps in the Building of a Dispersive NIR Spectrometer with an Application for Ethanol Quantification Using a Polymer Filament 3D Printer

The construction of a dispersive optical spectrometer using three-dimensional (3D) design software and printing, without applying any optical adjustments, its validation, and application to quantification of ethanol in multiproduct liquids, is the objective of this work. A 3D design software was used to design a near-infrared (NIR) spectrometer in the region from 800 to 1600 nm from the dimensions of commercially available optical components. The project was printed on a polymer filament 3D printer, and the components were fitted to the printed part. Software calculations using the model design parameters were applied to attribute the wavelength values to the abscissa axis in the spectra and estimate errors due to 3D printing limitations. The alignment of the spectrum was proven using the chloroform absorbance spectrum, which presented a maximum mispositioning of 4.1 nm concerning the literature data and effective bandwidths equivalent to commercial instruments. The 3D-printed instrument was applied to quantify ethanol in samples of cachaça, rum, beer, brandy, whiskey, vodka, mouth wash, alcohol gel, and commercial alcohol solutions. Partial least-squares regression models were built for the 3D-printed instrument and two commercial NIR instruments, the MPA II (Bruker) and the NIR DLP NIRscan (Texas Instruments), using a group of 180 standards. The three instruments reached excellent predictive capability with similar root mean square error of cross-validation (2.36-2.68) and prediction (2.31-2.87). The correlation coefficient of cross-validation and prediction for all models were between 0.97 and 0.98. The results show the feasibility of building a 3D-printed dispersive spectrometer ready for application with the simple docking of the optics, presenting acceptable accuracy to the project design concerning the printing limitations.

A thermoelectrically stabilized aluminium acoustic trap combined with attenuated total reflection infrared spectroscopy for detection of Escherichia coli in water

Acoustic trapping is a non-contact particle manipulation method that holds great potential for performing automated assays. We demonstrate an aluminium acoustic trap in combination with attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) for detection of E. coli in water. The thermal conductivity of aluminium was exploited to thermo-electrically heat and hold the acoustic trap at the desired assay temperature of 37 °C. Systematic characterisation and optimisation of the acoustic trap allowed high flow rates while maintaining high acoustic trapping performance. The ATR element serves not only as a reflector for ultrasound standing wave generation but also as a sensing interface. The enzyme conversion induced by alkaline phosphatase-labelled bacteria was directly monitored in the acoustic trap using ATR-FTIR spectroscopy. Sequential injection analysis allowed automated liquid handling, including non-contact bacteria retention, washing and enzyme-substrate exchange within the acoustic trap. The presented method was able to detect E. coli concentrations as low as 1.95 × 106 bacteria per mL in 197 min. The demonstrated ultrasound assisted assay paves the way to fully automated bacteria detection devices based on acoustic trapping combined with ATR-FTIR spectroscopy.

Polarimetric Balanced Detection: Background-Free Mid-IR Evanescent Field Laser Spectroscopy for Low-Noise, Long-term Stable Chemical Sensing

In this work, we introduce polarimetric balanced detection as a new attenuated total reflection (ATR) infrared (IR) sensing scheme, leveraging unequal effective thicknesses achieved with laser light of different polarizations. We combined a monolithic widely tunable Vernier quantum cascade laser (QCL-XT) and a multibounce ATR IR spectroscopy setup for analysis of liquids in a process analytical setting. Polarimetric balanced detection enables simultaneous recording of background and sample spectra, significantly reducing long-term drifts. The root-mean-square noise could be improved by a factor of 10 in a long-term experiment, compared to conventional absorbance measurements obtained via the single-ended optical channel. The sensing performance of the device was further evaluated by on-site measurements of ethanol in water, leading to an improved limit of detection (LOD) achieved with polarimetric balanced detection. Sequential injection analysis was employed for automated injection of samples into a custom-built ATR flow cell mounted above a zinc sulfide multibounce ATR element. The QCL-XT posed to be suitable for mid-IR-based sensing in liquids due to its wide tunability. Polarimetric balanced detection proved to enhance the robustness and long-term stability of the sensing device, along with improving the LOD by a factor of 5. This demonstrates the potential for new polarimetric QCL-based ATR mid-IR sensing schemes for in-field measurements or process monitoring usually prone to a multitude of interferences.