Silicon Microcantilever Sensors to Detect the Reversible Conformational Change of a Molecular Switch, Spiropyan

Abstract

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...

Photoactuation of micromechanical devices by photochromic molecules

“…a tensile strain), which leads to photoactuation. This finding is perfectly in line with the results of Grogan et al 22,23 reported for silicon cantilevers grafted by a spiropyran monolayer. These authors have observed also a downward (upward) cantilever deflection upon the SP to MC (MC to SP) isomerization.…”

“…10%), which they attributed to the lower elastic modulus of the photoisomerized polymer film. On the other hand, Grogan et al 22,23 have functionalized gold-coated silicon microcantilevers with a spiropyran monolayer by immersing the devices into the solution of the molecules. When irradiated by UV and visible light sources the microcantilever showed a reversible deflection, which was assigned to surface stress changes induced by the photoisomerization.…”

Direct Color Observation of Light‐Driven Molecular Conformation‐Induced Stress

“…[16,44] On the other hand, opposite responses were found in the case of gold-coated silicon cantilevers decorated with the spiropyran photoswitch as a function of the location of the SAM (silicon or gold side). [17,45] Thus, the direction of the bending may depend on a multitude of factors including the cantilever material and configuration as well as the intermolecular forces within the layer and between the molecules and the transducer surface. Regarding the PDMS-MCCs, however, an accurate comparison with the reported data might be difficult to attain since polymeric cantilevers in combination with DABSYL-Cl SAMs have not been investigated before.…”

“…and bio-chemical interactions. [10,11] For instance, the structural evolutions of the molecular switches topoisomerase II, [12] bacteriorhodopsin, [13] calmodulin, [14] DNA, [15] azobenzene, [16] and spiropyran [17] have been successfully interrogated with mechanical sensors.…”

Review: Cantilever-Based Sensors for High Speed Atomic Force Microscopy

“…Another variable that can be used to quantify the amount of the adsorbed gases in the microcantilever is the change in the resonant frequency caused by the surface stresses. However, when compared to the mass loading, the effects of such stresses on the resonant frequency are found to be insignificant [ 76 , 77 ].…”