A Ag/melanin/SS memristive device with remarkably good endurance and retention properties is demonstrated using natural melanin isolated from bacterium Aeromonas sp. SNS. Purified and characterized melanin is used for the development of a non‐volatile memory device. The device is operated under electroforming‐free mode and requires only ±0.6 V resistive switching voltage. 2 × 104 switching cycles endurance is achieved with an approximately 10X memory window at 0.2 V read voltage. In addition to this, the low resistance state (LRS) and high resistance state (HRS) are stable over 103 s without any observable degradation in the resistance states. Standard deviation and coefficient of variation of LRS and HRS are very small compared to other memory devices reported in the literature. Furthermore, the Ag/melanin/SS memristive device shows rich nonlinear behavior and possesses negative differential resistance effect during positive bias. Charge transportation in the device is due to the ohmic and space charge limited current. The formation and breaking of the conductive filament are responsible for the bipolar resistive switching effect due to hybrid electronic‐ionic charge transportation. Rich I–V characteristics, very low switching voltage, and remarkable nonvolatile memory performance make melanin a strong candidate for next generation organic non‐volatile memory devices.
Abstract3,4-Dihydroxy l-phenylalanine (l-DOPA) is considered a potent drug for the treatment of Parkinson disease. Physical and nutritional parameters where optimized by using Yarrowia lipolytica-NCIM 3450 to accomplished the highest production of l-DOPA. Screenings of critical components were completed by using a Plackett–Burman design, while further optimization was carried out using the Box–Behnken design. The optimized factor levels predicted by the model were pH 6.1, 1.659 g L−1 yeast extract, 1.491 g L−1l-tyrosine and 0.0290 g L−1 CuSO4. The predicted yield of l-DOPA with these levels was 1.319 g L−1, while actual yield obtained was 1.273 g L−1. The statistical analysis revealed that model is significant with F value 19.55 and R2 value 0.9514. This process resulted in a 3.594-fold increase in the yield of l-DOPA. l-DOPA was confirmed by HPTLC and HPLC analysis. Thus, Yarrowia lipolytica-NCIM 3450 has potential to be a new source for the production of l-DOPA.Graphical AbstractElectronic supplementary materialThe online version of this article (doi:10.1007/s13659-014-0017-3) contains supplementary material, which is available to authorized users.
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