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Herein we investigate the feasibility of detecting photo-induced surface stress changes using the deflection response of cantilevers. For this purpose, silicon microcantilevers have been functionalised with spiropyran photochromic molecules, using both a monolayer and a polymeric brushes approach. Upon ultraviolet light irradiation, the spiropyran unit is converted to the merocyanine form due to the photo-induced cleavage of the C spiro -O bond. The two forms of the molecule have dramatically different charge, polarity and molecular conformations. This makes spiropyrans an ideal system to study the correlation between photo-induced molecular changes and corresponding changes in surface stress. Our investigations include monitoring the changes in static cantilever deflection, and consequently, surface stress of the spiropyran functionalised cantilevers on exposure to ultraviolet light. Cantilever deflection data reveals that ultraviolet induced conformational changes in the spiropyran moiety cause a change in compressive surface 2 stress and this varies with the type of functionalisation method implemented. The change in surface stress response from the spiropyran polymer brushes functionalised cantilevers gives an average surface stress change of 98 Nm -1 (n = 8) while the spiropyran monolayer coated cantilevers have an average surface stress change of about 446 Nm -1 (n = 24) upon irradiation with UV light.

High-sensitivity and simple, low-cost readout are desirable features for sensors independent of the application area. Micro-cantilever sensors use the deflection induced by the analyte presence to achieve high-sensitivity but possess complex electronic readouts. Current holographic sensors probe the analyte presence by measuring changes in their optical properties, have a simpler low-cost readout, but their sensitivity can be further improved. Here, the two working principles were combined to obtain a new hybrid sensor with enhanced sensitivity. The diffractive element, a holographically patterned thin photopolymer layer, was placed on a polymer (polydimethylsiloxane) layer forming a bi-layer macro-cantilever. The different responses of the layers to analyte presence lead to cantilever deflection. The sensitivity and detection limits were evaluated by measuring the variation in cantilever deflection and diffraction efficiency with relative humidity. It was observed that the sensitivity is tunable by controlling the spatial frequency of the photopolymer gratings and the cantilever thickness. The sensor deflection was also visible to the naked eye, making it a simple, user-friendly device. The hybrid sensor diffraction efficiency response to the target analyte had an increased sensitivity (10-fold when compared with the cantilever or holographic modes operating independently), requiring a minimum upturn in the readout complexity.

The high sensitivity of silicon microcantilever sensors has expanded their use in areas ranging from gas sensing to bio-medical applications. Photochromic molecules also represent promising candidates for a large variety of sensing applications. In this work, the operating principles of these two sensing methods are combined in order to detect the reversible conformational change of a molecular switch, spiropyran. Thus, arrays of silicon microcantilever sensors were functionalized with spiropyran on the gold covered side and used as test microcantilevers. The microcantilever deflection response was observed, in five sequential cycles, as the transition from the spiropyran (SP) (CLOSED) to the merocyanine (MC) (OPEN) state and vice-versa when induced by UV and white light LED sources, respectively, proving the reversibility capabilities of this type of sensor. The microcantilever deflection direction was observed to be in one direction when changing to the MC state and in the opposite direction when changing back to the SP state. A tensile stress was induced in the microcantilever when the SP to MC transition took place, while a compressive stress was observed for the reverse transition. These different type of stresses are believed to be related to the spatial conformational changes induced in the photochromic molecule upon photo-isomerisation.Sensors 2020, 20, 854 2 of 12 areas ranging from gas, humidity and thermal sensing to novel applications in microbiology, genomics and cancer detection [9][10][11][12][13][14][15]. Innovative microcantilever coatings include polymer brushes-based on phenylboronic acid which have been used to detect glucose binding events [15] and graphene oxide (GO) thin films for high-sensitivity humidity sensing [16]. Other examples involve the use of microcantilever sensors for detection of various diseases through, for example, antibody-antigen interactions for the detection of disease-related C-reactive proteins or for the screening of heart-related diseases by using cardiomyocytes functionalized microcantilevers [17][18][19]. Different fabrication methods for microcantilevers as well as the range of available modification methods in the substrate and sensing layer allow for the realization of different types of cantilever-based sensors [20,21]. An improvement in microcantilevers sensor efficiency of the n-type over p-type silicon cantilevers was demonstrated, and this effect was explained by their greater piezoresistive coefficient [20]. Examples include micromachined silicon cantilever paddle sensors which can be used in high flow rate gas sensing [20,21], and resonant cantilevers for pressure sensing [22].Spiropyrans (SP) are one of the most popular families of photochromic molecules [23,24]. Upon irradiation with UV light, the orthogonal SP isomer converts to the planar merocyanine (MC) form due to the photo-cleavage of the C spiro -O bond. The MC isomer shows a strong absorption band in the visible region due to its conjugation. When the MC is exposed to visible light, the structu...