摘要
发展灵敏的、选择性的快速检测细菌芽孢的方法对于医疗诊断和人类健康治疗来说显得越来越重要。近年来,表面增强拉曼散射(SERS)技术因其具有的高灵敏度、高分辨率以及快速响应而在生物分子检测应用中吸引了众多研究工作者的兴趣。构建SERS基底的一个关键性的挑战是基底不仅需具备生物兼容性,而且还要能有力地实现理想的检测灵敏度和选择性。在本文中,我们设计了一种新型的基于金纳米粒子的SERS基底,并证明了其对细菌芽孢中释放出来的生物标志物的检测可以获得高灵敏度和低检测限。我们将在以下几个密切相关的领域中讨论主要的研究发现。
金纳米粒子/聚乙烯吡咯烷酮/金基底(AuNPs/PVP/Au)的制备及其SERS效应将被首次讨论(第2章)。关于Au NPs的粒径与SERS基底的增强效应两者间相互关系的研究工作表明：60nm金粒子制备的基底获得的SERS增强效应最大,增强因子可达～106。这一类型基底具备的强SERS效应来源于Au NPs-Au NPs及AuNPs-金基底间的相互作用所产生的双重耦合效应,我们还通过控制纳米粒子的尺寸范围(50-70nm)对其进行了优化。
该AuNPs/PVP/Au基底经实验证明可以实现高灵敏检测吡啶2,6-二羧酸(简称DPA,为细菌芽孢中的生物标志物)(第3章)。在考察DPA浓度与SERS特征峰之强度两者关系的工作中我们观察到了两个线性范围,“低浓度区域”(<0.01ppm)及“高浓度区域”(>1ppm),并且在这二者之间还存在着一个“过渡区域”。在SERS检测DPA的研究中,能在这样一个低浓度区观察到线性关系实属首例。实验结果证明利用60nm的AuNPs制备的基底可以获得0.1ppb的检测限,据我们所知该极限值为在此类SERS基底检测DPA的报道中最低值,低浓度的DPA在AuNPs/PVP/Au基底上获得的极大K值(1.7×107M-1)也可以证明这一发现。为了洞察DPA浓度与SERS强度间的相关性,我们依据“单分子层和多分子层吸附等温线”对DPA在SERS基底上的吸附特征进行了分析。我们认为实验中所观察到的介于低和高浓度区域之间的“过渡”对应于吸附等温线从单分子层吸附到多分子层吸附的“转变”,而且该“过渡区域”的实验数据与我们估算出来的满一个单分子层覆盖所需DPA的最低浓度理论值(-0.01ppm)十分吻合。
为了确定这一新型SERS基底的光谱鉴别和定量能力,也为了解生物标志物DPA在纳米粒子修饰的基底上是否可能发生表面反应,我们认为有必要考察生物标志物和其脱羧反应的可能性产物一吡啶(Py)的竞争吸附体系(第4章)。在实验数据分析中我们发现：DPA和Py两者虽然在它们的环呼吸振动模式相关的SERS光谱范围内(900-1100cm-1)显示出一些相似性,但是二者在AuNPs/PVP/Au基底上的独立吸附和竞争吸附的特征波谱中都表现出了明显差别。在实验检测的浓度范围内,Py在基底上获得的吸附平衡常数(～8×105)比DPA的(～1×105)大得多,原因是Py在Au表面上的绑定能力比DPA的强。实验结果不但为我们提供了本SERS检测条件下生物标志物在基于金纳米粒子的基底上物种形成的准确信息,而且也验证了此类基底在有表面反应存在或无表面反应存在的条件下实现高灵敏、高选择性检测细菌标志物的可行性。
通过分析pH和阴离子对样品溶液SERS强度的影响从而系统地研究了影响SERS检测AuNPs/PVP/Au基底上DPA的因素(第5章)。实验结果表明：SERS的强度会随着pH值的下降而上升,我们认为此现象反映了DPA在SERS基底上存在着吸附模式的“质子化—脱质子化”间的平衡。溶液中存在的不同阴离子也对SERS强度有着一定的影响,反映了不同阴离子在SERS基底上绑定能力的差异。
我们将上述SERS基底作为光谱探针检测枯草芽孢杆菌的芽孢,以达到进一步检验该基底性能的目的,实验结果证明：其对细菌芽孢中释放出来的标志物钙化DPA (CaDPA)也具有高灵敏性(第6章)。利用硝酸从芽孢中萃取出的CaDPA在SERS检测中获得了一系列特征光谱,并且得到了1.5×109spores/L(即：2.5x10-14M)的检测限(LOD)。将Au纳米粒子构建的SERS基底和报道中的Ag纳米粒子基底相比较不难发现：二者获得的LOD值非常接近,但是前者的表面吸附平衡常数比后者的小了1～2个数量级。上述这一系列的发现均证明了Au纳米粒子制备的SERS基底是可以直接作为高分辨率、高灵敏的生物兼容性探针而应用于细菌芽孢的检测。
此外,为了建立在不同条件下对细菌芽孢的SERS险测,我们又考察了芽孢的储备时间和检测环境对SERS检测的影响(第7章)。实验中以AuNPs/PVP/Au基底为探针检测到了芽孢在特定条件下“自释放”出的CaDPA,这一发现与文献报道中关于芽孢在控制温度和湿度的条件下可以追踪到CaDPA的释放是一致的。在关于激光功率对SERS强度的影响这一研究中发现：后者随着前者的增大而上升。这一发现证明：相比其他物种在不同激光功率下表现的不稳定性,从芽孢中释放出来的DPA/CaDPA在本实验条件的激光照射下是稳定的。
综上所述,本工作中构建的AuNPs/PVP/Au基底可以被确认为是检测枯草芽孢杆菌中生物标志物的高灵敏SERS基底,以此为基础则可进一步拓展和优化其在检测各种不同细菌芽孢方面的应用。
The development of sensitive, selective, and rapid methods for the detection of bacterial spores is increasingly important for medical diagnostics and human health care. The high sensitivity, high resolution, rapid response time of surface-enhanced Raman scattering (SERS) techniques have recently attracted a great deal of interest in biomolecular detection. A key challenge is the ability to establish the SERS substrate which is not only biocompitable but also robust for achieving the desired detection sensitivity and selectivity. In this dissertation work, a new type of gold nanoparticle-based substrate has been demonstrated for the detection of biomarkers released from bacterial spores with high sensitivity and low detection limit. Major findings are discussed in following closely-related aspects.
The preparation of gold-nanoparticle/polyvinylpyrrolidone/gold-substrate (AuNPs/PVP/Au) and its SERS effect are first described (Chapter2). The study of the correlation between the gold nanoparticle size and the SERS intensity has demonstrated that the SERS intensity is maximized with the size of60nm, displaying an enhancement factor of～106. The strong SERS effect of this type of substrate exploits both particle-particle and particle-substrate plasmonic coupling effects, which is optimized by manipulating the diameter of the nanoparticles in the range of50-70nm.
The AuNPs/PVP/Au substrate has been demonstrated to be viable for highly sensitive detection of dipicolinic acid (DPA), a biomarker from bacterial spores (Chapter3). The correlation between the SERS intensity of the diagnostic bands and the DPA concentration (0.1ppb-100ppm) is shown to exhibit two linear regions, i.e., the low-(<0.01ppm) and high-concentration (>1ppm) regions, with an intermediate region in between. The presence of a linear relationship in the low-concentration region has been observed for the first time in SERS detection of DPA. A detection limit of0.1ppb is obtained from the substrates with60-nm sized Au NPs, which is to our knowledge the lowest detection limit reported for DPA using this type of SERS substrate. This finding is consistent with the large adsorption equilibrium constant for the low-concentration region (1.7×107M-1). The adsorption characteristics of DPA on the SERS substrates has been analyzed in terms of monolayer and multilayer adsorption isotherms to gain insights into the correlation between the SERS intensity and the DPA concentration. The observed transition from the low-to high-concentration linear regions is shown to correspond to the transition from a monolayer to multilayer adsorption isotherm, which is in a good agreement with the estimated minimum DPA concentration for a monolayer coverage (～0.01ppm).
To further establish the spectroscopic identification and quantification capabilities of the new SERS substrate, possible reactivities of the biomarkers on the nanoparticle-based substrate have been examined by analyzing the SERS characteristics for the competitive adsorption of the biomarker DPA and pyridine (Py) which is a possible decarboxylation product of DPA (Chapter4). The analysis focuses on the diagnostic region of900-1100cm-1associated with the ring-breathing modes of the two molecules. While the SERS spectra in this region appeared to display some similarities between DPA and Py, distinctive differences in the detailed band characteristics were revealed for both individual and competitive adsorptions on the AuNPs/Au substrates. The fact that the equilibrium constant for the adsorption of Py on the substrate (-8×105M-1) is larger than that for DPA (～1×105M-1) in the measured concentration region is attributed to a stronger binding of Py to Au surface than that for DPA. The results have provided not only accurate speciation of the biomarker molecules on the gold nanoparticle based substrates under the SERS measurement conditions, but also has implications for expanding the application of the nanoparticle substrates for highly sensitive and selective detection of bacterial biomarkers under various reactive or non-reactive conditions.
Factors influencing the SERS detection of DPA on the AuNPs/PVP/Au substrate have also been systematically examined by studying pH and anion effects of the sample solution on the SERS intensity (Chapter5). The results have revealed a clear increase of SERS intensity with the decrease of pH, which is believed to reflect the protonation-deprotonation equilibrium of DPA on its adsorption modes on the SERS substrate. The presence of different anions in the solution has also been found to exhibit some influences on the SERS intensity, which is believed to reflect the difference in binding strength among the different anions upon adsorption on the SERS substrate.
The capability of the SERS substrate has further been investigated as a spectroscopic probe for the detection of Bacillus subtilis spores, demonstrating high sensitivity to the detection of biomarker calcium dipicolinate (CaDPA) released from the bacterial spores (Chapter6). The SERS bands of CaDPA released from the spores by extraction using nitric acid provide the diagnostic signal for the detection, exhibiting a limit of detection (LOD) of1.5×109spores/L (or2.5×10-14M). The LOD for the Au NP based substrates is quite comparable with that reported for Ag nanoparticle based substrates for the detection of spores, though the surface adsorption equilibrium constant is found to be smaller by a factor of1-2orders of magnitude than the Ag nanoparticle based substrates. These findings have demonstrated the viability of the gold nanoparticle-based SERS substrates for direct use with high resolution and sensitivity as a biocompatible probe for the detection of bacteria spores.
In an effort to establish the SERS detection of bacterial spores under various conditions, the effects of storage time of the bacterial spores and the detection environment on the SERS detection have also been investigated (Chapter7). The results have revealed that the spores could undergo "self-releasing" of CaDPA under certain conditions, a finding consistent with literature report on monitoring of CaDPA release from spores under controlled temperature and wet conditions. The examination of the effect of laser power on the SERS intensity revealed a systematic increase with the power. In contrast to the chemical instability of other chemicals under different laser powers, this finding has demonstrated the stability of the DPA/CaDPA released from the bacterial spores against the irradiation of laser power under our experimental conditions.
In summary, the AuNPs/PVP/Au substrates developed in this work has been concluded to function as a highly-sensitive SERS substrate for the detection of biomarkers released from Bacillus subtilis spores, which upon further optimization could find widespread applications for the detection of a variety of different bacterial spores.
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