Collaborative Research: An Integrated NMR Microcoil Detector for Microspectroscopy

核磁共振波谱是确定复杂化学和生化分子三维结构的最有效的方法之一。来自核磁共振的信息已被证明在确定蛋白质的一级、二级和三级结构以及研究酶机制和治疗药物的结合部位方面是必不可少的。它还广泛用于化学合成，能够确定多步合成中间步骤产物的结构和立体化学。尽管核磁共振技术具有巨大的潜力，但在细胞微分析化学分析和生物系统研究中经常遇到的纳升体积样品的应用一直受到限制。然而，我们团队在螺线管核磁共振微线圈设计方面的最新进展(Olson等人，1995)为在各种新的应用中应用核磁共振方法提供了基础。例如，我们已经开始研究在组合化学(Gordon等人，1994)中遇到的质量限量的核磁共振研究中的化学和生物学应用(Gordon等人，1994)，在从凝胶微滴中捕获单细胞进行生物制品分析(Weaver等人，1991)，以及作为纳升体积微电泳系统中的检测器(Fan和Harrison，1994；Jacobson等人，1994)。核磁共振技术在这些新应用中的主要缺点是，与其他分析方法相比，核磁共振固有的低灵敏度。这个问题往往因可获得的样本数量很少而变得更加复杂。这可能是由于：(I)可以合成或分离的量，(Ii)在合适的溶剂中的溶解度有限，或(Iii)在长时间的数据采集过程中降解。几乎每个商用高分辨率核磁共振谱仪都使用一个小的射频线圈，它安装在一个5 mm的熔融二氧化硅管周围，提供大约0.5毫升的有效细胞体积。人们很早就认识到这种射频(RF)线圈可以减小尺寸，但这种减小的优势直到最近才被证明。我们小组的研究表明，对于质量有限的样品，通过使用直径300-400微米的螺线管线圈，S/N比可以比传统的5 mm线圈提高100以上。通过进一步减小线圈直径，有可能进一步提高信噪比。然而，在这样的尺寸下制造螺线管变得非常有问题。然而，转向光刻制造技术打开了一扇新的机会之窗。主要的问题是显微镜平面线圈与光谱仪其余部分的有效接口。在我们最近的NSF SGER拨款中(NSF BIR93-19399，“用于核磁共振显微镜的单片砷化镓接收器”)，我们成功地设计和构建了建议的射频线圈/前置放大器系统的混合版本，用于7.05T下的1H-核磁共振应用研究。在核磁共振光谱实验中使用该混合原型获得的结果与理论预测非常一致，并证明了在细胞研究中使用有源单片探测器来提高检测性能的可行性。此外，为了进一步提高信噪比，我们制作了一个500 MHz的集成电路核磁共振探测器。这种设计可以很容易地扩展到更高的频率。这项拟议研究的目标是建立一系列可与现有的(250、300、500 MHz)和新兴的(750、1000 MHz)核磁共振显微镜系统接口的单片式砷化镓核磁共振接收器。我们将把这一设计扩展到宽带多级放大器配置，它将在100-500 MHz的频率范围内运行，消除了对接收线圈进行调谐和匹配的需要。我们建议使用一种综合的方法来设计这些探测器系统，其中包括电路模拟(微波设计软件，HP)、器件制造(化合物半导体微电子中心，UIUC)、电特性(S参数和噪声测量)和核磁共振评估(在每个目标磁场强度下)。

Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful methods available for determining the three dimensional structure of complex chemical and biochemical molecules. The information derived from NMR has proved essential in determining primary, secondary and tertiary structures of proteins, as well as investigating enzymatic mechanisms and the binding sites of therapeutic drugs. It is also used extensively in chemical synthesis, being able to determine structure and stereochemistry of the products of intermediate steps in a multi-stage synthesis. In spite of its great potential, the application of NMR techniques to the nanoliter volume samples often encountered in microanalytical chemical analysis and studies of biological systems at the cellular has been limited. However, recent advances in the design of solenoidal NMR microcoils by our group (Olson et al., 1995) provide a basis for applying NMR methods in a variety of new applications. For example, we have begun to investigate chemical and biological applications in NMR studies of the mass limited quantities encountered in combinatorial chemistry (Gordon et al., 1994), in biological product analysis from single cells entrapped in gel microdroplets (Weaver et al., 1991) and as detectors in nanoliter volume microelectrophoresis systems (Fan and Harrison, 1994; Jacobson et al., 1994). The major drawback in such new applications of NMR techniques is the intrinsic low sensitivity of NMR compared with other analytical methods. The problem is often compounded by the very small mass of sample which is available. This can be due to: (i) the quantity which can either be synthesized or isolated, (ii) limited solubility in suitable solvents, or (iii) degradation over the long data acquisition times. Almost every commercial high resolution NMR spectrometer uses a small RF coil which fits around a 5 mm fused-silica tube giving an effective cell volume of approximately 0.5 ml. It has long been recognized that this radiofre quency (RF) coil can be reduced in size, but the advantages of this reduction have not been demonstrated until recently. Our group showed that by using solenoidal coils of diameters 300-400 microns, improvements in S/N of over 100 can be achieved over a conventional 5 mm coil for mass limited samples. Further improvements in the SNR can potentially be made by reducing the coil diameter yet further. However, fabrication of solenoids at such dimensions becomes highly problematic. Switching to lithographic fabrication techniques, however, opens up a new window of opportunity. The major problems are efficient interfacing of the microscopic planar coils with the rest of the spectrometer. In our recent NSF SGER grant (NSF BIR 93-19399, "Monolithic Gallium Arsenide Receiver for NMR Microscopy") we successfully designed and constructed a hybrid version of the proposed RF coil/preamplifer system at 300 MHz for 1H-NMR application studies at 7.05 T. The results obtained using this hybrid prototype in NMR spectroscopy experiments agree well with theoretical predictions and demonstrate the feasibility of using active monolithic detectors for improved detection performance in cellular studies. In addition, we fabricated a 500 MHz integrated circuit NMR detector for further improvement in SNR. This design can be easily extended to higher frequencies. The goal of the proposed research is to build a family of monolithic GaAs NMR receivers that will interface with current (250, 300, 500 MHz) and emerging (750, 1000 MHz) NMR microscopy systems. We will extend this design to a broadband multistage amplifier configuration that will operate over a frequency range 100 - 500 MHz, eliminating the need for tuning and matching of the receive coil. We propose to design these detector systems using an integrative approach that involves circuit simulation (Microwave Design Software, HP), device fabrication (Center for Compound Semiconductor Microelectronics, UIUC), electrical characterization (S-parameter and noise measur ements), and NMR evaluation (at each of the target magnetic field strengths).