Collaborative Research: An Integrated NMR Microcoil Detector for Microspectroscopy

核磁共振（NMR）光谱是测定复杂化学和生物化学分子三维结构的最有力的方法之一。来自NMR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位点方面是必不可少的。它还广泛用于化学合成，能够确定多阶段合成中中间步骤产物的结构和立体化学。尽管其巨大的潜力，NMR技术的应用纳升体积的样品中经常遇到的微量化学分析和生物系统的研究在细胞一直是有限的。然而，我们小组在螺线管NMR微线圈设计方面的最新进展（Olson等人，1995）为在各种新应用中应用NMR方法提供了基础。例如，我们已经开始研究在组合化学中遇到的质量有限量的NMR研究中的化学和生物学应用（Gordon等人，1994），在来自凝胶微滴中截留的单细胞的生物产物分析中（Weaver等人，1991）和作为纳升体积微电泳系统中的检测器（Fan和Harrison，1994; Jacobson等人，1994年）。 NMR技术的这种新应用的主要缺点是与其他分析方法相比，NMR固有的低灵敏度。这个问题往往由于可获得的样品质量非常小而变得更加复杂。这可能是由于： (i)可以合成或分离的量， (ii)在合适溶剂中的溶解度有限，或（iii）在长的数据采集时间内降解。 几乎每一个商业高分辨率NMR光谱仪都使用一个小型RF线圈，该线圈安装在5 mm熔融石英管周围，有效池体积约为0.5 ml。人们早就认识到，这种射频（RF）线圈的尺寸可以减小，但这种减小的优点直到最近才得到证明。我们的小组表明，通过使用直径为300-400微米的螺线管线圈，对于质量有限的样品，S/N的改善超过100，可以超过传统的5 mm线圈。通过进一步减小线圈直径，可以潜在地进一步改善SNR。然而，在这样的尺寸下制造电容器变得非常成问题。然而，转向光刻制造技术打开了一扇新的机会之窗。主要问题是显微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资助（NSF BIR 93-19399，“用于NMR显微镜的单片砷化镓接收器”）中，我们成功地设计并构建了300 MHz的混合型RF线圈/前置放大器系统，用于7.05 T的1H-NMR应用研究。在NMR光谱实验中使用该混合原型获得的结果与理论预测吻合良好，并证明了在细胞研究中使用有源单片检测器提高检测性能的可行性。此外，我们制作了一个500 MHz的集成电路NMR检测器，以进一步提高信噪比。这种设计可以很容易地扩展到更高的频率。拟议的研究的目标是建立一个家庭的单片GaAs NMR接收器，将接口与当前（250，300，500 MHz）和新兴（750，1000 MHz）NMR显微镜系统。我们将这种设计扩展到一个宽带多级放大器配置，将在100 - 500 MHz的频率范围内工作，消除了调谐和匹配的接收线圈的需要。我们建议设计这些检测器系统使用一个综合的方法，涉及电路模拟（微波设计软件，惠普），设备制造（中心化合物半导体微电子，UIUC），电气特性（S-参数和噪声测量），和NMR评价（在每个目标磁场强度）。

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).