超声轴向传播技术评价长骨的研究

用超声轴向传播技术评价长骨状况已成为近年来的一个研究热点。本文首先介绍了第一到达波(FAS)法、超声Lamb波及柱面导波法在评价长骨皮质骨状况方面的研究进展,最后分析和讨论了当前研究中存在的问题及应该研究的方向。     

Joint spectrogram segmentation and ridge-extraction method for separating multimodal guided waves in long bones

Ultrasonic guided waves (GWs) can be used to evaluate long bones effectively because of the ability to provide the information of the whole bone. In this study, a joint spectrogram segmentation and ridge-extraction (JSSRE) method was proposed to separate multiple modes in long bones. First, the Gabor time-frequency transform was applied to obtain the spectrogram of multimodal signals. Then, a multi-class image segmentation algorithm was used to find the corresponding region of each mode in the spectrogram, including an improved watershed transform and a region growing procedure. Finally, the ridges were extracted and the time domain signals representing individual modes were reconstructed from these ridges in each region. The validations of this method were discussed by simulated multimodal signals with different signal-to-noise ratios (SNR). The correlation coefficients between the original signals without noise and the reconstructed signals were calculated to analyze the results quantitatively. The results showed that the extracted ridges were in good agreement with generated theoretical dispersion curves, and the reconstructed signals were highly related to the original signals, even under the SNR=3 dB situation.     

Cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase

ABSTRACT

The synthesis and characterisation of several members of the 1,ω-bis(4-cyanobiphenyl-4′-yl) alkane (CBnCB) and the 1-(4-cyanobiphenyl-4′-yloxy)-ω-(4-cyanobiphenyl-4′-yl) alkane (CBnOCB) homologous series are reported. The new odd members described CB5CB, CB13CB, CB4OCB, CB8OCB and CB10OCB all exhibit twist-bend nematic and nematic phases. The members of these series already reported in literature, CB7CB, CB9CB, CB11CB and CB6OCB, were also prepared in order to allow for a direct comparison of their transitional properties. The properties of these dimers are also compared to those of the corresponding members of the 1,ω-bis(4-cyanobiphenyl-4,-yloxy) alkanes (CBOnOCB). For any given total spacer length, for odd members of these series, the nematic–isotropic transition temperatures and associated entropy changes are greatest for the CBOnOCB dimer and lowest for the CBnCB dimer. These trends are understood in terms of molecular shape. For short spacer lengths, the twist-bend nematic–nematic transition temperature (T_NTBN) is higher for the CBnOCB series than for the CBnCB series but this is reversed as the spacer length increases. Of the CBOnOCB dimers, a virtual value of T_NTBN was estimated for CBO3OCB and T_NTBN was measured for CBO5OCB. These values are considerably lower than those observed for the corresponding members of the CBnCB or CBnOCB series. The dependence of T_NTBN on molecular structure is discussed not only in terms of the molecular curvature but also in the ability of the molecules to pack efficiently. As the temperature range of the preceding nematic phase increases, so the twist-bend nematic–nematic transition entropy change decreases and the transition approaches second order for the longer spacers. For comparative purposes, the transitional behaviour of the even-membered dimers CB6CB, CB5OCB and CBO4OCB is reported and differences accounted for in terms of molecular shape.     

Structural characteristics of different types of nanoparticles synthesised in mesomorphic metal alkanoates

The class of thermotropic ionic liquid crystals (LCs) of the metal alkanoates possesses a number of unique properties, such as intrinsic ionic conductivity, high dissolving ability and ability to form time-stable mesomorphic glasses. These ionic LCs can be used as nanoreactors for the synthesis and stabilisation of different types of nanoparticles (NPs). Thus, some semiconductors, metals and core/shell NPs were chemically synthesised in the thermotropic ionic liquid crystalline phase (smectic A) of the cadmium octanoate (CdC₈) and of the cobalt octanoate (CoC₈). By applying the scanning electron microscopy, the cadmium and cobalt octanoate composites containing CdS, Au, Ag and core/shell Au/CdS NPs have been studied. NPs’ sizes and dispersion distribution of the NPs’ size in the nanocomposites have been obtained.     

基于催化应用调控氧化铈纳米材料的形貌

催化剂的设计、合成和结构调控是获得优异性能的关键.传统的策略主要是尽量减小催化剂颗粒尺寸以增加活性中心的数目,即尺寸效应.近年来,材料科学的快速发展使得在纳米尺度上调变催化剂的尺寸和形貌成为可能,特别是通过形貌调控可暴露更多的高活性晶面,大幅度提高催化性能,即纳米催化中的形貌效应.因此,调节催化剂的尺寸与形貌可以单独或协同优化材料的性能.氧化铈作为催化剂的重要组分与结构、电子促进剂被广泛应用于多相催化剂体系.本文总结了近期氧化铈材料形貌可控合成的进展,包括主要的合成策略和表征方法; 进而分析了氧化铈和金-氧化铈催化材料的形貌效应,指出金-氧化铈之间独特的相互作用与载体形貌密切相关; 阐述了氧化铈纳米材料因暴露晶面的差异而获得不同催化性能的化学机制.     

Dual fluorescence and ultrafast intramolecular charge transfer with 6-N,N-dialkylaminopurines. A two-state model

6-N,N-Dimethyl-9-methyladenine (DMPURM) and 6-N,N-dimethyladenine (DMPURH) show dual fluorescence from a locally excited (LE) and an intramolecular charge transfer (ICT) state in solvents of different polarity over extended temperature ranges. The fluorescence quantum yields are very small, in particular those of LE. For DMPURM in acetonitrile (MeCN) at 25 °C, for example, Φ'(ICT) = 3.2 × 10(-3) and Φ(LE) = 1.6 × 10(-4). The large value of Φ'(ICT)/Φ(LE) indicates that the forward LE → ICT reaction is much faster than the back reaction. The data obtained for the intersystem crossing yield Φ(ISC) show that internal conversion (IC) is the dominant deactivation channel from LE directly to the ground state S(0). For DMPURM in MeCN with Φ(ISC) = 0.22, Φ(IC) = 1 - Φ(ISC) - Φ'(ICT) - Φ(LE) = 0.78, whereas in cyclohexane an even larger Φ(IC) of 0.97 is found. The dipole moment gradually increases upon excitation, from 2.5 D (S(0)), via 6 D (LE) to 9 D (ICT) for DMPURM and from 2.3 D (S(0)), via 7 D (LE) to 8 D (ICT) for DMPURH. From the temperature dependence of Φ'(ICT)/Φ(LE), a reaction enthalpy -ΔH of 11 kJ/mol is obtained for DMPURM in n-hexane (ε(25) = 1.88), increasing to 17 kJ/mol in the more polar solvent di-n-butyl ether (ε(25) = 3.05). With DMPURM in diethyl ether, an activation energy of 8.3 kJ/mol is determined for the LE → ICT reaction (k(a)). The femtosecond excited state absorption spectra at 22 °C undergo an ultrafast decay: 1.0 ps in CHX and 0.63 ps in MeCN for DMPURM, still shorter (0.46 ps) for DMPURH in MeCN. With DMPURM in n-hexane, the LE fluorescence decay time τ(2) increases upon cooling from 2.6 ps at -45 °C to 6.9 ps at -95 °C. The decay involves ICT and IC as the two main pathways: 1/τ(2) ? k(a) + k(IC). As a model compound (no ICT) is not available, its lifetime τ(0)(LE) ～ 1/k(IC) is not known, which prevents a separate determination of k(a). The excited state reactions of DMPURM and DMPURH are treated with a two-state model: S(0) → LE ? ICT. With 6-N-methyl-9-methyladenine (MPURM) and 9-methyladenine (PURM), the fluorescence quantum yield is very low (<5 × 10(-5)) and dominated by impurities, due to enhanced IC from LE to S(0).     

超声诊断骨质疏松症中松质骨的模型

骨质疏松症(OP)是老龄化社会中影响健康的一个重要问题，超声技术已成为诊断骨质疏松症的一种常用方法。文中综述了近年来用超声诊断骨质疏松症中松质骨模型研究的进展，对棒状模型、流体多孔介质模型(Biot理论)和层状模型(schoenberg理论)进行了分析和讨论，指出了各理论模型存在的缺陷，对下一步的研究工作提出了建议。     

Competitive phosphorylation of some β-cyclodextrin-based inclusion compounds with phosphorous diamidoester

Inclusion compounds of β-cyclodextrin with hexane-1-ol and 1-(4-isobutylphenyl)propanoic acid were subjected to competitive phosphorylation with phosphorous diamidoester derived from 1,2-O-isopropylideneglycerol to show that the reactions occur under mild conditions but with different efficiencies depending on the nature of the “guest.”     

Dynamics of ultrafast intramolecular charge transfer with 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) in n-hexane and acetonitrile

The intramolecular charge transfer (ICT) reaction of 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) in n-hexane and acetonitrile (MeCN) is investigated by picosecond fluorescence experiments as a function of temperature and by femtosecond transient absorption measurements at room temperature. NTC6 in n-hexane is dual fluorescent from a locally excited (LE) and an ICT state, with a quantum yield ratio Phi'(ICT)/Phi(LE) of 0.35 at +25 degrees C and 0.67 at -95 degrees C, whereas in MeCN mainly an ICT emission is observed. From the temperature dependence of Phi'(ICT)/Phi(LE) for NTC6 in n-hexane, an LE/ICT enthalpy difference DeltaH of -2.4 kJ/mol is determined. For comparison, 1-isopropyl-6-cyano-1,2,3,4-tetrahydroquinoline (NIC6) is also investigated. This molecule does not undergo an ICT reaction, because of its larger energy gap DeltaE(S1,S2). From the molar absorption coefficient epsilonmax of NTC6 as compared with other aminobenzonitriles, a ground-state amino twist angle theta of approximately 22 degrees is deduced. The increase of epsilonmax between n-hexane and MeCN indicates that theta decreases when the solvent polarity becomes larger. Whereas single-exponential LE fluorescence decays are obtained for NIC6 in n-hexane and MeCN, the LE and ICT decays of NTC6 in these solvents are double exponential. For NTC6 in n-hexane at -95 degrees C, with a shortest decay time of 20 ps, the forward (ka=2.5x10(10) s(-1)) and backward (kd=2.7x10(10) s(-1)) rate constants for the LE<-->ICT reaction are determined from the time-resolved LE and ICT fluorescence spectra. For NTC6 in n-hexane and MeCN, the excited-state absorption (ESA) spectrum at 200 fs after excitation is similar to the LE(ESA) spectra of NIC6 and 4-(dimethylamino)benzonitrile (DMABN), showing that LE is the initially excited state for NTC6. These results indicate that the LE states of NTC6, NIC6, and DMABN have a comparable molecular structure. The ICT(ESA) spectrum of NTC6 in n-hexane and MeCN resembles that of DMABN in MeCN, likewise indicating a similar ICT structure for NTC6 and DMABN. From the decay of the LE absorption and the corresponding growing-in for the ICT state of NTC6, it is concluded that the ICT state originates from the LE precursor and is not formed by direct excitation from S0, nor via an S2/ICT conical intersection. The same conclusion was made from the time-resolved (picosecond) fluorescence spectra, where there is no ICT emission at time zero. The decay of the LE(ESA) band of NTC6 in n-hexane occurs with a shortest time tau2 of 2.2 ps. The ICT reaction is much faster (tau2 = 0.82 ps) in the strongly polar MeCN. The absence of excitation wavelength dependence (290 and 266 nm) for the ESA spectra in MeCN also shows that LE is the ICT precursor. With NIC6 in n-hexane and MeCN, a decay or growing-in of the femtosecond ESA spectra is not observed, in line with the absence of an ICT reaction involving an S2/ICT conical intersection.     

一维链状配位聚合物[Cd{5-(NO2)sal}2(2,2′-bipy)]n的合成与晶体结构

A coordination polymer of [Cd{5-(NO₂)sal}₂(2,2′-bipy)]_n(5-(NO₂)sal=5-nitrosalicylate, 2,2′-bipy=2,2′-bipyridine) was synthesized by hydrothermal reaction and characterized by elemental analysis, IR and X-ray diffraction single crystal structure analysis. The title complex crystallizes in monoclinic with space group C2/c, a=2.730 8(16) nm, b=1.272 3(5) nm, c=0.674 5(3) nm, β=96.73(2)°, V=2.327(2) nm³, Z=4, R=0.022 2, wR=0.057. The 5-nitrosalicylate anions doubly bridge the Cd(Ⅱ) atoms to form one-dimensional polymeric chain with the repeated eight-membered ring units (Cd-O-C-O)₂. The crystal structure is stabilized by intra- and interchain hydrogen bonds interactions. CCDC: 694568.