Self-emitting blue and red EuOX (X = F, Cl, Br, I) materials: band structure, charge transfer energy, and emission energy.

Self-emitting blue and red EuOX (X = F, Cl, Br, and I) were successfully synthesized and characterized. Far-infrared and Raman measurements revealed that the vibration modes prominently reflected the Eu-O and Eu-X bond characters of these materials. X-ray photoelectron spectroscopy (XPS) of the red-emitting EuOX compounds showed that Eu exclusively existed as Eu3+, while in the blue-emitting EuOX, a mixed Eu3+/Eu2+ state was observed. For the red-emitting EuOX (X = F, Cl, and Br), the maximum wavelengths of the charge-transfer (CT) bands were red-shifted: F → Cl → Br (282, 320, and 330 nm for F, Cl, and Br, respectively). Using one-electron spin-polarized band structure calculations, it was verified that the red-shift of the CT energy from F to Br in EuOX was mainly due to the relative positions of the halogen orbital energies being gradually increased, following the trend in their electronegativity. For the blue-emitting EuOX (X = Cl, Br, and I), the emission band maxima were red-shifted from Cl to I (409, 414, and 432 nm for Cl, Br, and I, respectively), which was quite opposite to the trend predicted based on the spectrochemical series in crystal field theory, which was in good agreement with the previous results of the calculated 5d → 4f transition energies of the Eu2+ activator based on the crystal field theory. Through photoluminescence, UV-visible absorbance, and XPS, it was elucidated that the red emission due to Eu3+ was strongly masked by the intensified blue emission associated with the small amount of Eu2+ in the blue-emitting EuOX (X = Cl, Br, and I). These materials may provide a platform for modeling new phosphors for application in solid-state lighting.

Charge Sensitive Optical Detection for Measurement of Small-Molecule Binding Kinetics.

Charge sensitive optical detection (CSOD) technique is a label-free method for real-time measurement of molecular interactions. Traditional label-free optical detection techniques mostly measure the mass of a molecule, and they are less sensitive to small molecules. In contrast, CSOD detects the charge of a molecule, where the signal does not diminish with the size of the molecule, thus capable for studying small molecules. In addition, CSOD is compatible with the standard microplate platform, making it suitable for high-throughput screening of drug candidates. In CSOD, an optical fiber functionalized with the probe molecule is dipped into a well of a microplate where an alternate perpendicular electrical field is applied to the fiber, which drives the fiber into oscillation because of the presence of surface charge on the fiber. The binding of the target molecules changes the charge of the fiber, and thus the amplitude and phase of the oscillating fiber, which are precisely measured through tracking of the optical images of the fiber tip.

Charge-Sensitive Optical Detection of Binding Kinetics between Phage-Displayed Peptide Ligands and Protein Targets.

Phage display technology has been a powerful tool in peptide drug development. However, the supremacy of phage display-based peptide drug discovery is plagued by the follow-up process of peptides synthesis, which is costly and time consuming, but is necessary for the accurate measurement of binding kinetics in order to properly triage the best peptide leads during the affinity maturation stages. A sensitive technology is needed for directly measuring the binding kinetics of peptides on phages to reduce the time and cost of the entire process. Here, we show the capability of a charge-sensitive optical detection (CSOD) method for the direct quantification of binding kinetics of phage-displayed peptides to their target protein, using whole phages. We anticipate CSOD will contribute to streamline the process of phage display-based drug discovery.

Critical angle reflection imaging for quantification of molecular interactions on glass surface.

Quantification of molecular interactions on a surface is typically achieved via label-free techniques such as surface plasmon resonance (SPR). The sensitivity of SPR originates from the characteristic that the SPR angle is sensitive to the surface refractive index change. Analogously, in another interfacial optical phenomenon, total internal reflection, the critical angle is also refractive index dependent. Therefore, surface refractive index change can also be quantified by measuring the reflectivity near the critical angle. Based on this concept, we develop a method called critical angle reflection (CAR) imaging to quantify molecular interactions on glass surface. CAR imaging can be performed on SPR imaging setups. Through a side-by-side comparison, we show that CAR is capable of most molecular interaction measurements that SPR performs, including proteins, nucleic acids and cell-based detections. In addition, we show that CAR can detect small molecule bindings and intracellular signals beyond SPR sensing range. CAR exhibits several distinct characteristics, including tunable sensitivity and dynamic range, deeper vertical sensing range, fluorescence compatibility, broader wavelength and polarization of light selection, and glass surface chemistry. We anticipate CAR can expand SPR's capability in small molecule detection, whole cell-based detection, simultaneous fluorescence imaging, and broader conjugation chemistry.

Charge-Sensitive Optical Detection of Small Molecule Binding Kinetics in Normal Ionic Strength Buffer.

Most label-free detection technologies detect the masses of molecules, and their sensitivities thus decrease with molecular weight, making it challenging to detect small molecules. To address this need, we have developed a charge-sensitive optical detection (CSOD) technique, which detects the charge rather than the mass of a molecule with an optical fiber. However, the effective charge of a molecule decreases with the buffer ionic strength. For this reason, the previous CSOD works with diluted buffers, which could affect the measured molecular binding kinetics. Here, we show a technique capable of detecting molecular binding kinetics in normal ionic strength buffers. An H-shaped sample well was developed to increase the current density at the sensing area to compensate the signal loss due to ionic screening at normal ionic strength buffer, while keeping the current density low at the electrodes to minimize the electrode reaction. In addition, agarose gels were used to cover the electrodes to prevent electrode reaction generated bubbles from entering the sensing area. With this new design, we have measured the binding kinetics between G-protein-coupled receptors (GPCRs) and their small molecule ligands in normal buffer. We found that the affinities measured in normal buffer are stronger than those measured in diluted buffer, likely due to the stronger electrostatic repulsion force between the same charged ligands and receptors in the diluted buffer.