A Cucurbit[8]uril 2:2 Complex with a Negative pKa Shift

A viologen derivative carrying a benzimidazole group ( V-P-I ²⁺; viologen–phenylene–imidazole V-P-I ) can be dimerized in water using cucurbit[8]uril (CB[8]) in the form of a 2:2 complex resulting in a negative shift of the guest pK_a, by more than 1 pH unit, contrasting with the positive pK_a shift usually observed for CB-based complexes. Whereas 2:2 complex protonation is unclear by NMR, silver cations have been used for probing the accessibility of the imidazole groups of the 2:2 complexes. The protonation capacity of the buried imidazole groups is reduced, suggesting that CB[8] could trigger proton release upon 2:2 complex formation. The addition of CB[8] to a solution containing V-P- I ³⁺ indeed released protons as monitored by pH-metry and visualized by a coloured indicator. This property was used to induce a host/guest swapping, accompanied by a proton transfer, between V-P-I ³⁺ ⋅ CB[7] and a CB[8] complex of 1-methyl-4-(4-pyridyl)pyridinium. The origin of this negative pK_a shift is proposed to stand in an ideal charge state, and in the position of the two pH-responsive fragments inside the two CB[8] which, alike residues engulfed in proteins, favour the deprotonated form of the guest molecules. Such proton release triggered by a recognition event is reminiscent of several biological processes and may open new avenues toward bioinspired enzyme mimics catalyzing proton transfer or chemical reactions.     

氮化镓单晶生长研究进展

氮化镓(GaN)作为第三代宽禁带半导体核心材料之一,具有高击穿场强、高饱和电子漂移速率、抗辐射能力强和良好的化学稳定性等优良特性,是制作宽波谱、高功率、高效率光电子、电力电子和微电子的理想材料.受制于氮化镓单晶衬底的尺寸、产能及成本的影响,当前的GaN基器件主要基于异质衬底(硅、碳化硅、蓝宝石等)制作而成,GaN单晶衬底的缺乏已成为制约GaN器件发展的瓶颈.近年来,国内外在GaN单晶衬底制备方面取得了较大的进展.本文综述了氮化镓单晶生长的最新进展,包括氢化物气相外延法、氨热法和钠助熔剂法的研究进展,分析了各生长方法面临的挑战与机遇,并对氮化镓单晶材料的发展趋势讲行了展望.     

Optical image asymmetric cryptosystem using fingerprint based on iterative fraction Fourier transform

A novel asymmetric cryptosystem for optical image is proposed using fingerprint based on iterative fractional Fourier transform. To enhance the security, a hyperchaotic phase generated by a 4D Lorenz system is considered as the public key in the proposed encryption system, while the private key is emerged by the retrieved phase and fingerprint. In the encryption process, the secret information is hid into the hyperchaotic phase. Subsequently, the private key can be obtained by a reversible operation. To decrypt the original image, the ciphertext and private key are imported into the input plane of fractional Fourier system. This system is also applicable for information authentication because the fingerprint is used both in encryption and decryption approach. Some numerical simulations have been done to test the validity and capability of the encryption system.     

Co/SBA-16:对柴油组分有高选择性的费托合成催化剂(英文)

使用浸渍法制备了一系列Co/SBA-16催化剂,并通过氮物理吸附、X射线衍射、氢程序升温还原、氢化学吸附和透射电镜技术对催化剂行了表征。研究表明,随着负载钴含量的增加,金属钴的分散度降低了,这是同Co3O4晶粒增长和比表面积降低相一致的。Co/SBA-16催化剂展示了高的一氧化碳转化率,低的C1选择性和高的C5+选择性,特别对柴油组分有高选择性。     

超导磁共振仪器设备国产化现状及挑战

磁共振在化学分析和医学影像等领域发挥着不可或缺的作用，而磁共振仪器设备是开展磁共振研究的必要前提.长期以来，国外仪器厂商在我国磁共振仪器市场居于垄断地位.近年来，随着我国在磁共振仪器研发和产业化方面不断取得进展，市场份额为外商垄断的局面已大为改观.本文调研综述了我国磁共振仪器设备研制的现状，以及面临的若干挑战.     

A modified model for isothermal homogeneous and heterogeneous reactions in the boundary-layer flow of a nanofluid

The homogeneous and heterogeneous reactions in the boundary-layer of a flat surface are considered. The autocatalysts are assumed to be of regular sizes, while the solution is a dilute nanofluid. The heat release due to the chemical reactions is taken into account. The Buongiorno's model is used to describe the behaviors of this reaction system. This configuration makes the current model be different from all previous publications. Multiple solutions are given numerically to the rescaled nonlinear system, whose stability is verified. The results show that the strength coefficients of the homogeneous and heterogeneous reactions are key factors to cause the appearance of the multiple solutions in the distribution of the chemical reactions. Nanofluids enhance the diffusion of heat and help maintain the stability of chemical reactions.     

Primary and subharmonic simultaneous resonance of fractional-order Duffing oscillator

Nonlinear Dynamics - The primary and subharmonic simultaneous resonance of Duffing oscillator with fractional-order derivative is studied. Firstly, the approximately analytical solution of the...     

The stabilizability of heat equations with time-dependent coefficients

In this paper, we study the stabilizability of heat equations with time-dependent coefficients. The instability of the systems is illustrated by special solutions. The boundary feedback stabilization results are established by the backstepping method where the growth order of the kernel function depends on time.