Effect of thermal treatment on chitosan CDs in the solid state causes a redshift of PL emission and changes in surface states.

In this research, we have synthesized carbon dots (CDs) co-doped with nitrogen and sulfur by facile hydrothermal method, using citric acid and cysteine as carbon source. The effect of solid-state thermic treatment (STT) at 303 to 453 K on the size, surface, fluorescence and cellular cytotoxicity of the CDs were systematically investigated. Through a simple STT, it was possible to tune surface states and the average size of the CDs, causing a permanent red shift. Initially, CDs showed a decrease in cell viability with increasing concentration. However, after STT, its viability remained constant with an increase in concentration. Here, we show the possibility to label the cells cytoplasm according to the CDs fluorescence emission before (blue emission) and after STT (red emission). The CDs studied in this paper show selective luminescence properties, which are fundamental for any cell imaging application.

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The aim of this study was to obtain a Brazilian red propolis (BRP) enriched composite resin and to perform the characterization of its antibacterial activity, mechanical, and physical-chemical properties. Brazilian red propolis ethyl acetate extract (EABRP) was characterized by LC-ESI-Orbitrap-FTMS, UPLC-DAD, antibacterial activity, total flavonoids content, and radical scavenging capacity. BRP was incorporated to a commercial composite resin (RC) to obtain BRP enriched composite at 0.1, 0.15 and 0.25% (RP10, RP15 and RP25, respectively). The antibacterial activity RPs was evaluated against Streptococcus mutans by contact direct test and expressed by antibacterial ratio. The RPs were characterized as its cytotoxicity against 3T3 fibroblasts, flexural strength (FS), Knoop microhardness (KHN), post-cure depth (CD), degree of conversion (DC%), water sorption (Wsp), water solubility (Wsl), average roughness (Ra), and thermal analysis. Were identified 50 chemical compounds from BRP extract by LC-ESI-Orbitrap-FTMS. EABRP was bacteriostatic and bactericide at 125 and 500 μg/ml, respectively. The RP25 exhibited antibacterial ratio of 90.76% after 1 h of direct contact with S. mutans (p < 0.0001) while RC no showed significative antibacterial activity (p = 0.1865), both compared with cell control group. RPs and RC no showed cytotoxicity. RPs exhibited CD from 2.74 to 4.48 mm, DC% from 80.70 to 83.96%, Wsp from 17.15 to 21.67 μg/mm3, Wsl from 3.66 to 4.20 μg/mm3, Ra from 14.48 to 20.76 nm. RPs showed thermal resistance between 448–455°C. The results support that propolis can be used on development of modified composite resins that show antibacterial activity and that have compatible mechanical and physical-chemical properties to the indicate for composite resins.

The pH value is an important parameter as it is part of several processes, whether environmental or biological. In this report, S, N self-doped carbon dots (CDs) were synthesized by a simple hydrothermal method using cysteine (cys) and citric acid as precursors for a detailed investigation of size, morphological, photoluminescent, and structural changes at different pH values and its use as pH sensor and fluorescent ink. The fluorescence intensity of cys-CDs was dependent on the pH, presenting a linear relationship with pH values in the range of 2.0–9.0. Using spectroscopic techniques, a mechanism for the pH-dependent fluorescence is proposed, based on the aggregation of cys-CDs and also protonation/deprotonation of surface functional groups that change the excited state. The cys-CDs were found to be efficient as fluorescent pH sensors using real samples (distilled water and tap water). Furthermore, the pH changes in cys-CDs can be used for the visual enhancement of anti-counterfeiting technologies. Thus, the results of this study show that cys-CDs can act as an efficient and pH sensitive fluorescent sensor, which can be used to measure the pH value of water samples, due to its high fluorescence intensity, and can be applied successfully as a fluorescent ink.

In this study, we report the effect of the proportion of CdSe/CdS ultrasmall quantum dots (USQDs) conjugated on reduced graphene oxide (rGO). The proposed conjugation used a bond between the hydroxyl groups of the USQDs and the carboxylic groups of the rGO. The formation of the CdSe/CdS-rGO nanocomposites was confirmed by Fourier transform infrared spectroscopy (FTIR), high-resolution transmission electron microscopy (HRTEM), fluorescence (FL), cyclic voltammetry (CV), and photocurrent response versus time (PR). The FTIR results confirmed the bond between the hydroxyl groups present in the CdSe/CdS USQDs with the carboxyl groups of the rGO and with a higher USQD proportion, and the interactions also occur in the epoxide and hydroxyl groups. HRTEM images confirmed the FTIR results. FL spectra of the nanocomposite exhibited a luminescence quenching with USQD proportion. The time-resolved fluorescence observed that the nonradiative decay rate is due to the electron transfer process, reducing lifetime. CV experiments showed that the USQD proportion increases on the rGO-promoted better reversibility of the peak of the Fe 3+ /Fe 2+ system. PR results showed a very weak photocurrent on the rGO when illuminated at 375 nm that increased with the increase of the nanocomposite. Therefore, we have demonstrated that it is possible to tune the optical and electrical properties of the rGO-CdSe/CdS USQD nanocomposites.

Recently new methodologies for imaging have been achieved making use of multiple light scattering. Here we present the self-healing effect using a speckled light field. We present an experiment that constitutes a useful application for a three-dimensional light sheet-based imaging system through an inhomogeneous medium. Each layer can be imaged independently of the others. The axial resolution basically depends on the coherence length, which can be sub-wavelength and controllable. This allows for a simple and direct technique for imaging through scattering layers with axial resolution improvement. Our results may find applications not only in bio-microscopy systems but also in data transmission.