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Conference Paper
Palma, SICJ, Esteves C, Pádua AC, Alves CM, Santos GMC, Costa HMA, Dionisio M, Gamboa H, Gruber J, Roque ACA.  2019.  Enhanced gas sensing with soft functional materials, May 2019. ISOEN 2019 - 18th International Symposium on Olfaction and Electronic Nose, Proceedings. , Fukuoka, Japan: Institute of Electrical and Electronics Engineers Inc. AbstractPDF

The materials described in this work result from the selfassembly of liquid crystals and ionic liquids into droplets,
stabilized within a biopolymeric matrix. These systems are
extremely versatile gels, in terms of composition, and offer
potential for fine tuning of both structure and function, as
each individual component can be varied. Here, the
characterization and application of these gels as sensing thin
films in gas sensor devices is presented. The unique
supramolecular structure of the gels is explored for molecular
recognition of volatile organic compounds (VOCs) by
employing gels with distinct formulations to yield
combinatorial optical and electrical responses used in the
distinction and identification of VOCs.

Journal Article
Ferreira, IMPLVO, Eça R, Pinho O, Tavares P, Pereira A, Roque AC.  2007.  Development and Validation of an HPLC/UV Method for Quantification of Bioactive Peptides in Fermented Milks. Journal of Liquid Chromatography & Related Technologies. 30:2139–2147., Number 14 Abstract

The simultaneous separation and quantification of two casein peptides {(IPP}, {VPP)} presenting potent inhibitory activity of angiotensin-converting-enzyme {(ACE)} and casein in fermented milks was developed. Gradient elution was carried out at a flow-rate of 1 {mL/min}, using a mixture of two solvents. Solvent A was 0.1% {TFA} in water and solvent B was acetonitrile-water-trifluoracetic acid 95:5:0.1. The effluent was monitored by {UV} detector at 214 nm. Calibration curves were constructed in the interval of 0.01-1.0 {mg/mL} for {VPP}, 0.005-1.0 {mg/mL} for {IPP}, and 0.05-3.0 {mg/mL} for casein. R2 invariably exceeded 0.999. The detection limits were 0.004 for {VPP}, 0.002 {mg/mL} for {IPP}, and 0.02 {mg/mL} for casein. Repeatability of the method was evaluated by six consecutive injections of two standard solutions containing {VPP}, {IPP}, and casein. The {RSD} values for concentration were all below 5.08%. Recovery studies were carried out to determine the accuracy of the method. Recoveries ranged between 88 and 98.2%. The methodology was applied, not only, for the monitorization of {VPP}, {IPP}, and casein in commercial fermented milks labeled as presenting antihypertensive properties, but also, in milk with different degrees of fermentation by L. Helveticus, and in other commercial functional fermented milks, such as, those presenting cholesterol lowering properties.

Esteves C, Santos GMC, Alves C, Palma S, Porteira AR, Filho J, HA C, Alves VD, Faustino BMM, Ferreira I, Gamboa H, Roque ACA.  2019.  Effect of film thickness in gelatin hybrid gels for artificial olfaction. Materials Today Bio. 1:-. AbstractPDFWebsite

Artificial olfaction is a fast-growing field aiming to mimic natural olfactory systems. Olfactory systems rely on a first step of molecular recognition in which volatile organic compounds (VOCs) bind to an array of specialized olfactory proteins. This results in electrical signals transduced to the brain where pattern recognition is performed. An efficient approach in artificial olfaction combines gas-sensitive materials with dedicated signal processing and classification tools. In this work, films of gelatin hybrid gels with a single composition that change their optical properties upon binding to VOCs were studied as gas-sensing materials in a custom-built electronic nose. The effect of films thickness was studied by acquiring signals from gelatin hybrid gel films with thicknesses between 15 and 90 μm when exposed to 11 distinct VOCs. Several features were extracted from the signals obtained and then used to implement a dedicated automatic classifier based on support vector machines for data processing. As an optical signature could be associated to each VOC, the developed algorithms classified 11 distinct VOCs with high accuracy and precision (higher than 98%), in particular when using optical signals from a single film composition with 30 μm thickness. This shows an unprecedented example of soft matter in artificial olfaction, in which a single gelatin hybrid gel, and not an array of sensing materials, can provide enough information to accurately classify VOCs with small structural and functional differences.

Esteves, C, Ramou E, Porteira ARP, Barbosa AJM, Roque ACA.  2020.  Seeing the Unseen: The Role of Liquid Crystals in Gas‐Sensing Technologies. Advanced Optical Materials. 1902117:1-29. AbstractPDF

Fast, real-time detection of gases and volatile organic compounds (VOCs) is
an emerging research field relevant to most aspects of modern society, from
households to health facilities, industrial units, and military environments.
Sensor features such as high sensitivity, selectivity, fast response, and low
energy consumption are essential. Liquid crystal (LC)-based sensors fulfill
these requirements due to their chemical diversity, inherent self-assembly
potential, and reversible molecular order, resulting in tunable stimuliresponsive soft materials. Sensing platforms utilizing thermotropic uniaxial
systems—nematic and smectic—that exploit not only interfacial phenomena,
but also changes in the LC bulk, are demonstrated. Special focus is given to
the different interaction mechanisms and tuned selectivity toward gas and
VOC analytes. Furthermore, the different experimental methods used to
transduce the presence of chemical analytes into macroscopic signals are discussed and detailed examples are provided. Future perspectives and trends
in the field, in particular the opportunities for LC-based advanced materials in
artificial olfaction, are also discussed.

Moreira, IP, Esteves C, Palma SICJ, Ramou E, Carvalho ALM, Roque ACA.  2022.  Synergy between silk fibroin and ionic liquids for active gas-sensing materials, jun. Materials Today Bio. 15:100290.: Elsevier AbstractPDFWebsite

Silk fibroin is a biobased material with excellent biocompatibility and mechanical properties, but its use in bioelectronics is hampered by the difficult dissolution and low intrinsic conductivity. Some ionic liquids are known to dissolve fibroin but removed after fibroin processing. However, ionic liquids and fibroin can cooperatively give rise to functional materials, and there are untapped opportunities in this combination. The dissolution of fibroin, followed by gelation, in designer ionic liquids from the imidazolium chloride family with varied alkyl chain lengths (2–10 carbons) is shown here. The alkyl chain length of the anion has a large impact on fibroin secondary structure which adopts unconventional arrangements, yielding robust gels with distinct hierarchical organization. Furthermore, and due to their remarkable air-stability and ionic conductivity, fibroin ionogels are exploited as active electrical gas sensors in an electronic nose revealing the unravelled possibilities of fibroin in soft and flexible electronics.

Esteves, C, Palma SICJ, Costa HMA, Alves C, Santos GMC, Ramou E, Carvalho AL, Alves V, Roque ACA.  2022.  Tackling Humidity with Designer Ionic Liquid-Based Gas Sensing Soft Materials, dec. Advanced Materials. 34:2107205., Number 8: John Wiley & Sons, Ltd AbstractPDFWebsite

Relative humidity is simultaneously a sensing target and a contaminant in gas and volatile organic compound (VOC) sensing systems, where strategies to control humidity interference are required. An unmet challenge is the creation of gas-sensitive materials where the response to humidity is controlled by the material itself. Here, humidity effects are controlled through the design of gelatin formulations in ionic liquids without and with liquid crystals as electrical and optical sensors, respectively. In this design, the anions [DCA]− and [Cl]− of room temperature ionic liquids from the 1-butyl-3-methylimidazolium family tailor the response to humidity and, subsequently, sensing of VOCs in dry and humid conditions. Due to the combined effect of the materials formulations and sensing mechanisms, changing the anion from [DCA]− to the much more hygroscopic [Cl]−, leads to stronger electrical responses and much weaker optical responses to humidity. Thus, either humidity sensors or humidity-tolerant VOC sensors that do not require sample preconditioning or signal processing to correct humidity impact are obtained. With the wide spread of 3D- and 4D-printing and intelligent devices, the monitoring and tuning of humidity in sustainable biobased materials offers excellent opportunities in e-nose sensing arrays and wearable devices compatible with operation at room conditions.

Padrão, I, Fernandes CSM, Esteves C, Fernandes T, Pina AS, Roque ACA.  2020.  Versatile and Tunable Poly(Ethylene Glycol)‐Based Hydrogels Crosslinked through the Ugi Reaction. ChemPlusChem. 85(12):2737-2741.
Esteves, C, Palma S, Costa H, Alves C, Santos G, Ramou E, Roque AC.  2022.  VOC Sensing in Humid and Dry Environments, may. 2022 IEEE International Symposium on Olfaction and Electronic Nose (ISOEN). :1–3.: IEEE AbstractPDFWebsite

We report the development of gas-sensing multicomponent hybrid materials to be used under humidified and dried environments without the need of sample preconditioning or heavy signal processing. The easy tunability and the unique characteristics presented by the multicomponent hybrid materials suggests their use in nearterm applications in electronic nose systems able to operate in dry or humidified environments.