Handheld NMR Systems and Their Applications for Biomolecular Sensing - Point-of-Care Diagnostics on a Chip - page 175

This chapter explains the construction and experiment of our miniature NMR
systems. Its organization is as follows. Sections 8.2 and 8.3 present the design and
measurement of the silicon RF transceivers. Section 8.4 reports NMR experiments
and NMR-based biomolecular sensing. Section 8.5 compares our miniature NMR
systems to other NMR miniaturization efforts. We recommend readers unfamiliar
with NMR to read our publication of [ 2 ] for quick introduction to NMR basics
relevant to our work.

8.2 NMR RF Transceiver IC Design

8.2.1 Overall Architecture and Operation

We focus our discussion on the transceivers in the palm and 1-chip systems
(Figs. 8.2 and 8.3 ), for they are more advanced than the transceiver in the 2-kg
portable system (Fig. 8.1 ). Figure 8.4 a and b shows the architectures of the NMR
RF transceivers in the palm and 1-chip systems, respectively. The palm system
(Fig. 8.4 a) uses an off-chip solenoidal coil; the 1-chip system (Fig. 8.4 b) employs an
on-chip planar spiral coil. The dashed lines in the figures indicate silicon integration
boundaries for the two systems. The transceiver architecture is essentially the same
between the two systems, but the transceiver-coil matching networks are different
for a reason explained in Sect. 8.2.4 ; thus, the two separate figures were prepared
to avoid confusion. The electrical characteristics of the coils will be described in
Sect. 8.2.4 . All NMR experiments in our work, including biomolecular sensing, are
done with protons in hydrogen atoms in aqueous samples. In the palm system, a
sample is placed inside the solenoidal coil and is subjected to a static magnetic field
B 0 of 0.56 T produced by the ping-pong-ball-sized magnet. In the 1-chip system,
a sample placed on the planar coil is subjected to a static magnetic field B 0 of
0.49 T produced by the hamburger-sized magnet. The NMR frequency for protons
subjected to is given by ! 0 =2 D 42:6 B 0 MHz: 23.9 MHz for the palm system

and 20.9 MHz for the 1-chip system.
In the excitation phase of NMR, switches S1 and S2 are closed, and the
transmitter (upper half of Fig. 8.4 a or b) sends in an RF current to the coil to
produce an RF magnetic field in the sample. If the RF magnetic field's frequency is
tuned into the NMR frequency, ! 0 , it resonantly excites the protons, increasing their
energy. During this excitation phase, the receiver amplifier stages (in the lower half
of Fig. 8.4 a or b), except the front-end stage, are isolated from the large excitation
signal by short-circuiting their inputs and open-circuiting the RF signal path, using
switches S3 through S11 controlled by the ENA command signal. The front-end
stage remains connected to the large excitation signal, in order not to place switches
in front of it, as lossy switches at the front end would compromise the receiver
noise figure.
After protons acquire sufficient energy, the RF transmission is ceased by turning
off switches S1 and S2. Nearly simultaneously, the receiver path (lower half of