Development of Spiropyran Immobilization and Characterization Protocols for Reversible Photopatterning of SiO2 Surfaces.

Spiropyran is a dynamic organic compound that is distinguished by its reversible conversion between two forms: the colorless closed spiropyran (SP) form and the purple open merocyanine (MC) form. Typically triggered by UV light and reversed by visible light, spiropyran-functionalized surfaces offer reversible conversion in properties including color, polarity, reactivity, and fluorescence, making them applicable to diverse applications in chemical sensors, biosensors, drug delivery, and heavy metal extraction. While spiropyran has been successfully incorporated into various material platforms with SiO2 surfaces, its application on flat surfaces has been limited due to surface area constraints and a lack of standardized evaluation methods, which largely depend on the integration approach and substrate type used. In this study, we systematically review the existing literature and categorize integration methods and substrate types first and then report on our experimental work, in which we developed a streamlined three-step immobilization protocol, which includes surface activation, amination with (3-aminopropyl) triethoxysilane (APTES), and subsequent functionalization with carboxylic spiropyran (SP-COOH). Using SiO2 surfaces as a demonstration, we have also established a robust characterization protocol, consisting of contact angle measurements, X-ray photoelectron spectroscopy (XPS), ellipsometry, and fluorometric analysis. Our results evaluate the newly developed immobilization protocol, demonstrating effective activation and optimal amination using a 2% APTES solution, achieved in 5 min at room temperature. Fluorescence imaging provided clear contrast between the SP and the MC forms. Furthermore, we discuss limitations in the surface density of functional groups and steric hindrance and propose future improvements. Our work not only underscores the versatility of spiropyran in surface patterning but also provides optimized protocols for its immobilization and characterization on SiO2 surfaces, which may be adapted for use on other substrates. These advancements lay the groundwork for on-chip sensing technologies and other applications.

Corrigendum: Van der Waals interaction in uniaxial anisotropic media (2013 J. Phys.: Condens. Matter 25 035102).

A multiplication error led to incorrect values of C2e and C1o in Table 1 of the above paper. The correct table is given below. All numerical results, graphs, and conclusions of the paper remain unchanged. .

Ferromagnetism and Borromean binding in three-fermion clusters.

A three-particle spin-12 fermion problem with on-site repulsion and nearest-neighbor attraction is solved on the two-dimensional square lattice by discretizing a Schrödinger equation in momentum space. Energies of bound complexes (trions) and their binding conditions are obtained. For total spin S=1/2, a wide region of trion instability toward decaying into a stable singlet pair plus a free fermion is identified. The instability is attributed to the formation of a wave function node upon addition of the third fermion. In the S=3/2 sector, trions are found to form in the absence of bound pairs indicating Borromean binding. In the strong coupling limit the system transitions from an S=1/2 ground state to a ferromagnetic S=3/2 ground state in agreement with the Nagaoka theorem for a four-site plaquette.

One-dimensional model of inertial pumping.

A one-dimensional model of inertial pumping is introduced and solved. The pump is driven by a high-pressure vapor bubble generated by a microheater positioned asymmetrically in a microchannel. The bubble is approximated as a short-term impulse delivered to the two fluidic columns inside the channel. Fluid dynamics is described by a Newton-like equation with a variable mass, but without the mass derivative term. Because of smaller inertia, the short column refills the channel faster and accumulates a larger mechanical momentum. After bubble collapse the total fluid momentum is nonzero, resulting in a net flow. Two different versions of the model are analyzed in detail, analytically and numerically. In the symmetrical model, the pressure at the channel-reservoir connection plane is assumed constant, whereas in the asymmetrical model it is reduced by a Bernoulli term. For low and intermediate vapor bubble pressures, both models predict the existence of an optimal microheater location. The predicted net flow in the asymmetrical model is smaller by a factor of about 2. For unphysically large vapor pressures, the asymmetrical model predicts saturation of the effect, while in the symmetrical model net flow increases indefinitely. Pumping is reduced by nonzero viscosity, but to a different degree depending on the microheater location.

Van der Waals interaction in uniaxial anisotropic media.

Van der Waals interactions between flat surfaces in uniaxial anisotropic media are investigated in the nonretarded limit. The main focus is the effect of nonzero tilt between the optical axis and the surface normal on the strength of the van der Waals attraction. General expressions for the van der Waals free energy are derived using the surface mode method and the transfer-matrix formalism. To facilitate numerical calculations a temperature-dependent three-band parameterization of the dielectric tensor of the liquid crystal 5CB is developed. A solid slab immersed in a liquid crystal experiences a van der Waals torque that aligns the surface normal relative to the optical axis of the medium. The preferred orientation is different for different materials. Two solid slabs in close proximity experience a van der Waals attraction that is strongest for homeotropic alignment of the intervening liquid crystal for all the materials studied. The results have implications for the stability of plate-like colloids in liquid crystal hosts.

Electrochemical fabrication of nanodimensional multilayer films.

A novel method of fabricating nanodimensional multilayer films using electrochemistry is described. A thin layer of tantalum (Ta) is sputtered on a smooth insulating substrate. Ta is partially electrochemically oxidized (anodized) forming a Ta(2)O(5) layer. The rate of Ta consumption, the rate of Ta(2)O(5) expansion, and the dependence of Ta(2)O(5) thickness on anodization conditions have been carefully characterized to enable accurate predictions of the resulting thicknesses of both layers. Due to strong planarization action of the anodization process, the resulting interfaces Ta/Ta(2)O(5) and Ta(2)O(5)/electrolyte are remarkably smooth. The next layer of Ta is deposited on top of Ta(2)O(5), and the process is repeated as many times as needed. The Ta(2)O(5) layers are amorphous and pinhole free. We report fabrication of 10-layer structures with pitches ranging from 200 nm down to 12 nm and with excellent uniformity between the layers. The smallest achieved thickness of Ta layers is only 2.8 +/- 0.1 nm. The edges of such films, after proper polishing and etching, could serve as templates in nanoimprint lithography and in other applications.

Single-molecule designs for electric switches and rectifiers.

A design for molecular rectifiers is proposed. Current rectification is based on the spatial asymmetry of a molecule and requires only one resonant conducting molecular orbital. Rectification is caused by asymmetric coupling of the orbital to the electrodes, which results in asymmetric movement of the two Fermi levels with respect to the orbital under external bias. Results from numerical studies of the family of suggested molecular rectifiers, HS-(CH(2))(n)-C(6)H(4)(CH(2))(m)SH, are presented. Current rectification ratios in excess of 100 are achievable for n = 2 and m > 6. A class of bistable stator-rotor molecules is proposed. The stationary part connects the two electrodes and facilitates electron transport between them. The rotary part, which has a large dipole moment, is attached to an atom of the stator via a single sigma bond. Electrostatic bonds formed between the oxygen atom of the rotor and hydrogen atoms of the stator make the symmetric orientation of the dipole unstable. The rotor has two potential minima with equal energy for rotation about the sigma bond. The dipole can be flipped between the two states by an external electric field. Both rotor-orientation states have asymmetric current-voltage characteristics that are the reverse of each other, so they are distinguishable electrically. Theoretical results on conformation, energy barriers, retention times, switching voltages, and current-voltage characteristics are presented for a particular stator-rotor molecule. Such molecules could be the base for single-molecule switches, reversible diodes, and other molecular electronic devices.