Collaborative Research: An Integrated NMR Microcoil Detector for Microspectroscopy

核磁共振波谱是测定复杂化学和生化分子三维结构最有效的方法之一。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位点方面是必不可少的。它也广泛用于化学合成，能够确定多阶段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，但它在微分析化学分析和细胞生物系统研究中经常遇到的纳升体积样品中的应用受到了限制。然而，我们小组在螺线形核磁共振微线圈设计方面的最新进展（Olson et al., 1995）为将核磁共振方法应用于各种新应用提供了基础。例如，我们已经开始研究在组合化学中遇到的质量有限数量的核磁共振研究中的化学和生物应用（Gordon et al., 1994），在凝胶微滴包裹的单细胞生物产品分析中（Weaver et al., 1991），以及在纳升体积微电泳系统中作为检测器（Fan and Harrison, 1994; Jacobson et al., 1994）。与其他分析方法相比，核磁共振技术在这些新应用中的主要缺点是其固有的低灵敏度。可获得的样品质量非常小，这往往使问题更加复杂。这可能是由于：(i)可以合成或分离的数量，（ii）在合适溶剂中的溶解度有限，或（iii）在较长的数据采集时间内降解。几乎每个商用高分辨率核磁共振光谱仪都使用一个小的射频线圈，它适合大约5毫米的熔融硅管，有效细胞体积约为0.5毫升。人们早就认识到，这种射频（RF）线圈可以缩小尺寸，但这种缩小的优势直到最近才得到证明。我们的团队表明，通过使用直径300-400微米的螺线管线圈，在质量有限的样品中，比传统的5毫米线圈的信噪比可以提高100以上。进一步提高信噪比可以通过进一步减小线圈直径来实现。然而，在这样的尺寸制造螺线管变得非常有问题。然而，转向平版印刷制造技术打开了一个新的机会之窗。主要问题是显微镜平面线圈与光谱仪其余部分的有效连接。在我们最近的NSF SGER基金（NSF BIR 93-19399）中，“用于核磁共振显微镜的单片砷化镓接收器”)，我们成功地设计并构建了300 MHz射频线圈/前置放大器系统的混合版本，用于7.05 t的1H-NMR应用研究。使用该混合原型在核磁共振光谱实验中获得的结果与理论预测一致，并证明了在细胞研究中使用有源单片探测器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检测器。这种设计可以很容易地扩展到更高的频率。拟议研究的目标是建立一个家族的单片砷化镓核磁共振接收器，将与当前（250,300,500 MHz）和新兴（750,1000 MHz）核磁共振显微镜系统接口。我们将此设计扩展到宽带多级放大器配置，该配置将在100 - 500 MHz的频率范围内工作，从而消除了调谐和匹配接收线圈的需要。我们建议使用集成方法设计这些探测器系统，包括电路仿真（微波设计软件，HP），器件制造（化合物半导体微电子中心，UIUC），电气表征（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).