Development of quartz crystal oscillators for under-liquid sensing

Sensors and Actuators A, 29 (1991) 59-69 59

Development of quartz crystal oscillators for under-liquid sensing

Christopher Barnes Institute of Molecular and Biomokcular Electronics, School of EIectronic Engineering Science, University of Wales. Bangor Gwynedd LL57 IUT (l3.L)

(Received October 9, 1990, in revised form February 19. 1991; accepted March 22, 1991)

Abstract

A brief history of crystal sensing techniques is presented. Simple resonator equivalent circuits are outlined and expanded upon for cases of crystal immersion in liquids. Conditions for oscillation and choice of appropriate oscillator are discussed. Several well-known recently published sensor oscillator circuits are briefly reviewed and some suggestions for improvements are made. Details are given of two new oscillator circuits developed and employed by the author. Uniquely, these new circuits allow total immersion of the exposed crystal plate in hitherto impossible media, such as viscous liquids of viscosity up to 40 CP and in electrolytes and buffers of several millimolar ionic strength. Finally, suggestions for gain and stability control of oscillators in practical ‘under-liquid’ sensing situations are made in relation to the future potential of these techniques.

1. Introduction

I. I. Hktory Quartz crystals in the hundreds of kilohertz

and l-20 MHz ranges have long been used as sensitive monitors of mass [l] by monitoring the effects that this parameter has upon the crystal resonance frequency. More recently temperature [2] has been monitored in a similar manner.

It is not the intention here to discuss mass or temperature sensing in any great detail, since mass sensing is a well-established principle and only relatively simple oscillator circuits are required for its operation. Tem- perature sensing is more recently established, but is already highly commercialized and some excellent integrated microprocessor-based measurement units based on quartz crystals exist, particularly for temperatures up to 600 “C. More recently developed experimental topics in AT-crystal sensing will form the bulk of the discussion here.

For instance, it has been shown that it is possible to use AT-cut crystals to measure the viscosity of non-conducting organic liquids [3] and even to employ them as anaesthetic gas sensors [4] or in electrochemical experiments [5,6]. Indeed very recently and perhaps

most unexpectedly, aqueous biosensing [7] has been exploited.

With such a diversity of applications, it is expected that no one oscillator circuit will be universally suitable. Whilst several types of oscillator are commonly available, very few if any of them are absolutely ideal for the applications for which they have been employed, with workers choosing a particular design and often being ‘stuck with it’.

1.2. Crystal and oscillator theory Quartz is a piezoelectric material, i.e., when

an alternating electric field is placed across the relevant crystallographic plane(s), resulting alternating mechanical stresses are generated. In the case of the AT-cut plate the electric field is usually set up between small round electrodes on either face of the slice, which may be circular or square. This perpendicular field results in thickness shear motion of the surfaces of the slice. This motion occurs predominantly under the electrode. The series resonance frequency of the crystal is given by the standard resonance equation

where L and C are electrical equivalents of mechanical parameters, see Fig. 1 and below.

0924-4247/91/$3.50 Q 1991 - Elsevier Sequoia, Lausanne

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mechanical

i

L

c

R

Fig. 1. AT-resonator, standard equivalent circuit.

The mechanical resonance frequency fs is proportional to the reciprocal of the thickness d. The mechanical behaviour can be expressed electrically in terms of a series equivalent circuit where L, C and R are functions of the mass, compliance and internal friction losses respectively. The usual equivalent circuit of an AT-crystal resonator is shown in Fig. 1.

The AT-crystal in common with all piezoelectric resonators has the added complication that it exhibits dual resonance as a conse- quence of its packaging capacitance, a true electrical capacitance. Thus a parallel resonance frequencyf, is also possible at a slightly higher frequency than fs. Strictly, C, in Fig. 1 arises because of the dielectric constant of the quartz and the packaging capacitance via the air and the crystal connecting leads both within and outside the case/can (in most of these applications the crystal is usually operated with its can removed). In a circuit where neither side of the crystal is grounded, various stray capacitances to earth will feature in C,. At parallel resonance, the crystal offers a high terminal impedance, thus any suitable oscillator must be able to satisfy this condition. In the series mode the crystal has 0” phase change across its terminals and offers a very low resistance, being close to R, which is sometimes referred to as the e.s.r. (equivalent series resistance) or its motional resistance. When the crystal is used for mass measurement, small increases AL, occur in the series inductance L. When the crystal is exposed on one side to any liquid, small changes in all the series parameters will probably occur, but traditionally only mass loading has been .considered. With total immersion in a liquid, the situation will be as for one-sided exposure

in terms of effects on the mechanical resonance, but the parallel arm of the equivalent circuit will also be more strongly influenced. For exposure to a non-conducting liquid the crystal wires will effectively form capacitor plates and a change +AC, in C, will be expected to occur via the medium dielectric constant. It is expected that this change will only be detected if a parallel mode oscillator is employed. If a conducting liquid is employed, then an overall shunt impedance 2, will also occur, which may, in very simple terms, be thought of as contributing a AC,, effect and a parallel resistivity expected to give rise to additional effects, which further lower the crystal resonance frequency and broaden the Q-curve. Thus certain oscillators may appear non-ideal and hence the con- clusion may be, and it has been deduced by some workers [3], that crystals do not or cannot oscillate under conducting liquids. With the provision of sufficient gain and variable phase-shift in the oscillator-amplifier feedback path, it should at least in theory be possible to overcome this effect partially. Of course, when the solutions become very conductive oscillation may cease due to noth- ing other than a short-circuiting of the potential difference across the crystal and hence a collapse of the resulting piezo-field. It is possible that when a parallel-mode oscillator is employed under these conditions, it will be unable to sustain the normal operating point and its output frequency may first flick to the lower series resonance frequency before ultimate cessation of oscillation. Such effects have been observed experimentally by the author and are perceived as being due to additional phase-shift effects brought on by a combination of solution conductivity, dielectric constant and double-layer capacitance as a result of the above high-conductivity scenario. Possible equivalent circuits for loaded AT-resonators are shown in Fig. 2.

It follows from the equivalent circuit described in Fig. 1 that oscillators can be designed to excite either mode of oscillation of the crystal. Essentially an oscillator is an amplifier with its output connected in a loop to its input. The condition for oscillation is called the Barkhausen criterion and states that the total loop gain must exceed unity

L+AL

C+AC

R+AR

(4 6

?

Z.

L +AL

L+AC

B

CP AC, Rs

R+AR

cc)

Fig. 2. AT-resonator liquid-loaded equivalent circuits: (a) total immersion in an insulating dielectric liquid, AC,=0 for one-sided exposure; (b) total immersion in a conducting liquid; (c) simplest possible component breakdown of(b). In all three cases AL, AC, AR are due to viscoelastic mechanical loading.

and the total loop phase-shift must be 0”. In practice gains of several dB are required with AT-crystal to overcome the e.s.r. and the stiction, i.e., the non-linear amplitude re- sponse of the crystal mechanical oscillatory motion with applied driving power. Drive levels of 2-150 PW are typical with existing crystal oscillator designs for satisfactory operation.

Oscillators must be further designed to satisfy the correct phase relationships and terminal impedances. In order to do so, ac- count must be taken of both shifts introduced by devices and matching networks. For sensing applications, series-mode oscillators are generally preferable because the crystal is sometimes located remotely [B, 9] from the oscillator electronics. If this is not possible, then the effect of stray parallel capacitance should be kept to a minimum. If a parallel-mode oscillator must be employed, the connecting leads between the crystal sensor head should be kept as short and rigid as possible.

2. Factors which influence the choice of oscillator

It will be recognized that the choice of

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oscillator ought to be strongly influenced by the application in mind. For two reasons this has not exclusively been the case to date: first, due to the limited but ‘popular’ available designs; and secondly, possibly due to the lack of engineering backgrounds of some of the users and advocates of those designs. It is hoped that the work presented here will be particularly useful to scientists with more chemical or biological backgrounds.

The easiest conditions to deal with from the point of view of oscillator choice and design are sensors in which, although the crystal housing is removed, the crystal electrode surfaces (usually gold or silver) remain dry. This is the case in temperature and mass sensing, where with the latter only metallic or similar vapour deposition occurs and the crystal is operated within a vacuum chamber. In mass sensing, sensitivities of 1 ng are typical and sensitivities as low as 0.1 ng are not impossible. The sensitivity here is limited only by the tolerance and stability of the crystal frequency to other factors such as temperature and vibration relative to the changes in frequency Af brought about by the applied mass loading. In the microbalance case [l] both parallel- and series-mode oscillators have been successfully employed and even very simple low-dissipation one-transistor oscillators such as the Pierce, Hartley or Colpitts circuits are used to minimize self-induced thermal drift. Such circuits are so well known and well documented that they will not be discussed here, save to say that they are not suitable for applications of low crystal Q (see below). O’Dell [B] has recently suggested a low-dissipation Meacham bridge configura- tion which is equally suitable for mass sensing and a number of more sophisticated applications. Being series mode it has the advantage that the crystal can be operated quite remotely from the oscillator and independently of the effects of stray capacitance.

In some more specialized applications, such as electrochemical measurement, one side of the crystal should ideally be grounded in order to facilitate d.c. measurements against a reference electrode. As far as the author is

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aware, the most popular series circuit which satisfies this condition is that employed by Melroy et al. [9]_ However, Bruckenstein and Shay [5] succeeded with an oscillator where neither side of the crystal was grounded, by employing opto-isolation.

In areas of more recent application, such as viscosity measurement [3], electrochemistry [5, 63 and most recently biosensing [7], the equivalent ‘mass’ and viscous drag on the electrode area in shear are much more severe than in simple temperature or microbalance measurement, often resulting in a drastic lowering of the crystal & value. At very low Q values, the resulting oscillation has been described as ‘marginal’ [7]. Q is lowered even further by additional mass or viscous loading when the specialized additional layers required for some biosensing applications are either physically adsorbed [4] or chemically bound [7] to the crystal electrode surface. Several oscillator circuits have been published for such uses, but with few exceptions they have used the parallel mode. These parallel- mode circuits are suitable for driving crystals under most high-load conditions but suffer from the drawback that AC,, is highly variable, particularly in an electrochemistry or biosensing experiment where total coverage of one face of the crystal in liquid occurs, al- lowing stray capacitance to earth to exist. Also in an electrochemistry experiment an extra source of capacitive loading will be the d.c. connecting leads to the electrodes, although this can be kept to a minimum if choke/capacitor isolation is used.

Nomura and Okuhara [3] have recently shown that it is possible to make an AT-cut quartz crystal oscillate when totally immersed under organic liquids. It is believed, however, that to date no one has ever had success with total immersion under conducting solutions, such as electrolytes and buffers. The author’s circuits change this situation, as they are even suitable for weak solutions of the latter; their details will be given later. For all workers in biosensing and electrochemistry there has also up to now been the practical necessity, considered here to be a drawback, of one-sided crystal immersion, which has often involved having to remove the crystal plate from its mountings and affixing it to special cells or holders [5-71. Whether this has been as a

result of Nomura and Okuhara’s observation that total crystal immersion in a conducting liquid was ‘impossible’ remains to be seen. What is the case is that when both sides of an AT-cut crystal are immersed in any liquid, the frequency shifts due to viscous loading are significantly raised,. although the situation may not be quite so bad in comparison to one-sided operation, because the elaborate mounting devices and O-rings employed in some workers’ specialist cells with the latter probably give an equivalent load of their own, by bearing down a circumferential pressure on the quartz only just beyond the active electrode regions. Such additional loading is not present in the author’s system, where the crystal remains on its original mount and may be plugged into a conventional holder prior to immersion. Seeking to develop this system with the mass-produced AT-crystal as a dis- posable sensor head with potential medical and industrial implications, the author in- vestigated initially some of the currently available oscillator designs, in case they could be suitably modified to work with the crystal totally immersed in liquids.

The oscillator systems tested are therefore now reviewed in more detail, namely the circuits of Bruckenstein and Shay [5], Thomp- son et al. [7], O’Dell [8], Nomura and Okuhara [3] and Melroy et al. [9].

3. Review of circuits tested by the author

3.1. The Bruckenstein circuit This circuit, Fig. 3, is essentially a pair of

inverting buffers coupled in the linear mode and used as an amplifier, the crystal com- pleting the feed-back loop. Bruckenstein and Shay [S] have used the oscillator for elec-

Fig. 3. Essential features of Bruckenstein oscillator; author’s modification marked *.

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trochemical measurement. They state that the oscillator is ‘designed for parallel-resonant AT-cut crystals’, but since the total phase- shift through both gates is zero, it will excite the series resonant mode.

The most useful features of the Bruck- enstein circuit are its high output voltage and its ability to drive crystals under quite high load conditions. The author has had success with the circuit employing 14.333 MHz AT- cut silver electrode crystals oscillating totally immersed in glycerol-water mixtures up to 4 CP viscosity. Chokes and decoupling capacitors could theoretically be used to couple electrochemical voltammetry apparatus to a crystal without the need for optoisolation, if separate floating earth power supplies are employed. For temperature- or mass-sensing applications where great precision is required, a voltage-stabilized supply line would be required. However, in applications where the crystal stiction function is altered, for instance, with adsorbed layers as in biosensing, the provision of a variable supply voltage is extremely useful as it can be adjusted until the most stable operation is achieved. The author has experimented with control circuitry along these lines and some of these ideas will be expanded upon later.

3.2. The Thompson circuit Thompson et al. have described their os-

cillator as a ‘marginal oscillator’. It applies automatic gain control in order to increase the oscillator stage (40673) gain as the crystal Q diminishes. The 40673 operates as a Pierce- crystal oscillator, i.e., in the parallel resonant mode. It has been successfully employed by Thompson et al. for biosensing, namely pre- liminary studies of antibody-antigen inter- actions and liquid-crystal inter-facial effects where just one side of the crystal was exposed to liquids in a special O-ring-sealed flow- through cell [7]. The circuit was tested by the author, but found not to have sufficient gain for total crystal immersion, which is one of the goals of the author’s current research programme. Ideas for modification of this circuit to give increased gain will be discussed later. Advantages of the Thompson circuit as it stands are the provision of internally stabilized supply lines and the TIL output op- tion.

3.3. The circuit employed by Nomura and Okuhara

Nomura and Okuhara [3] employed a modified oscillator kit (Amtron International) for their studies of organic solution viscosity and density. Their circuit has the advantage of one-transistor simplicity, although it is not necessarily well optimized since the input impedance conditions at a common-emitter amplifier with low value earthy-end base bias chain resistor favour neither particular mode of operation, series or parallel. However, in order for the feedback to be positive, i.e., sustain oscillation, the transistor must look as though it is operating in common base to the feedback signal applied at the emitter. Thus this would favour the series mode of operation, since the return path would then be through the crystal es-r. to earth. In the paper it was concluded that the dielectric constant of the solutions did not effect the crystal’s operating frequency, despite its total immersion. Theoretical [lo] and practical con- siderations [ll] would tend to suggest this is not the case if the parallel mode had been excited, thus giving support for the series- mode operation hypothesis.

Since only one transistor is employed, the gain of this oscillator will be insufficient for marginal operation, and additional buffering will be required to operate a counter without frequency pulling.

3.4. The Melroy oscillator circuit This circuit comprises essentially a third

overtone series-mode oscillator circuit employing a video amplifier chip. The author initially tried unsuccessfully to duplicate this circuit by veroboard construction. However, it did function when a printed circuit board was designed and populated, and when supply rail decoupling capacitors were included. The circuit theoretically should be ideal for electrochemical/cyclic voltammetry applications since one side of the crystal is grounded. However, the gain was quite marginal and loaded crystals soon ‘died’ where the same crystals carried on oscillating merrily in the Bruckenstein circuit. Tuning of the 5 MHz inhibiting circuit was also very critical.

In the light of the poor results obtained with this circuit, the author decided to design and construct a series-mode circuit with one

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side of the crystal Crete components later.

grounded employing dis- which will be described

3.5. The O’Dell circuit In theory the O’Dell [8] circuit is an ex-

cellent series-mode oscillator. Since the crystal forms part of a bridge network, it is possible to choose the component values to balance the crystal e.s.r. perfectly at series resonance. The author built two versions of the circuit, one employing the chip specified and one using discrete components. The discrete components version, using BSX20 transistors throughout, excelled. Various modifications were made which allowed the crystal to work under water and electrolytic solutions (see Section 4). The only drawback of the O’Dell circuit as it was published is its low output voltage, = 100 mV r.m.s. and its fixed component values, limiting the choice of crystal frequency and e.s.r. The approximate in- equality equations [8] which define the stable oscillation criteria form a useful guide for practical modifications (see below).

4. Modifications to existing circuits

The only circuits which the author tried modifying were those of Bruckenstein and Shay [5], Thompson et al. [7] and O’Dell [8].

4.1. Modification to the Bruckenstein circuit As stated earlier, the Bruckenstein circuit

was modified at the output by means of an emitter follower. It was also possible to make it tuneable between 8 and 17 MHz by applying a 100 pF variable capacitor to ground at a point labelled * in Fig. 3.

4.2. Modifications to the Thompson circuit The Thompson circuit was modified as

shown in Fig. 4 by inserting two extra gain blocks in the form of cascaded BC184 transistors, each operating in common base. It was also found useful to apply the control line voltage onto the r-f. active gate of the MOSFET via the additional 1 Ma resistor. Also the voltage regulators were removed. The resulting circuit has sufficient gain to operate 18 MHz AT-cut crystals even with adsorbed surface layers under water and weak

Fig. 4. Author’s modifications to the Thompson circuit. Rest of circuit stays as before. X,=14-20 MHz.

ionic solutions. Both the above circuits were supply voltage sensitive, as appears to be the case experimentally in all sensing systems that employ either one-sided or total liquid immersion of the crystal. That is, the crystal stiction function will be very uncertain_ This is not as serious a problem as first expected, for in general it is possible to adjust the supply line voltage for the most stable operating point; drifts as low as a few tens of Hz per hour are possible at or close to this point, it being easy to detect when the operating point is wrongly set, since the drift increases to =2 kHz/hour! Some workers have discussed long-term drift in terms of water adsorption, but the author feels that this may be only a small part of the story in the light of the effects obtained on operating points above. It is perhaps interesting to note that a multiplicity of operating points has also recently been observed for altered crystal electrode patterns [12].

4.3. Modifications to the O’Dell circuit The O’Dell circuit was modified initially

by making all the resistors in the bridge and the collector load resistors variable. This en- abled any value of e.s.r. to be perfectly bal- anced and a whole range of crystals between 1 and 20 MHz could be made to oscillate in air or totally immersed in water, S-10 mM tris-HCl pH 7 solutions and NaCl solution of similar molarity. An extra transistor class A amplifier was added after the buffer to raise the output voltage to =4 V r.m.s. Total

65

immersion biosensing operations with a 14.333 MHz silver electroded AT-cut crystal were then begun using the resulting oscillator. It was employed for a while, but the manual resetting of the highly critical bridge and collector load resistors for stable oscillation during experimentation was rather cumber- some; eventually it was abandoned in favour of other designs. Note also that when using this oscillator with liquid-immersed crystals, it is best to include a d.c. blocking capacitor of 1 nF in series with the crystal to prevent electrolysis. This was also found to be the case for any oscillators in which significant d.c. potential occurs across the crystal.

5. The author’s designs

5.1. The parallel circuit The parallel oscillator circuit employed by

the author is shown in Fig. 5. It was powered by a stabilized but variable power supply unit. It employs a Phillips or Mullard HEF4049BP CMOS buffer chip which has extremely high gain, each gate consisting internally of three cascade MOS stages. (Note that chips from other manufacturers are not suitable for the design. Attempts to use them will result in an oscillator which will work in air but not under liquid immersion.)

Essentially the design is an extremely high- gain chip version of the Pierce oscillator. Similar circuits are commonplace for driving 32 kHz watch crystals and some temperature- sensing crystals [2], both of which have extremely high motional resistance up to 150

lOPI

1.7 - 20 ki oscillator buffer

Fig. 5. Author’s parallel-mode (high-gain Pierce) oscillator circuit.

kR. A 10 MHz unloaded AT-cut crystal typically has an e.s.r. of a few tens of ohms, rising to several hundred ohms for lower frequency (N 1 MHz) plates. 14 MHz loaded AT-crystals for marginal operation may have an es-r. as high as 500 or 1000 R [13]. Such high-gain parallel oscillators can cope easily with the diminished Q which results. Since the oscillator employed here is parallel mode, AC, must be kept to a minimum and during immersion experiments the oscillator was housed in a small die-cast box held by a clamp-stand, with the crystal holder pro- truding on stiff wire legs, sealed in a bakelite tube. The tip of the tube, holder and decanned crystal could thus readily be immersed in a beaker of the test liquid. The oscillator was tested under viscous load conditions using 14.333 MHz AT-cut silver electroded crystals and would even oscillate at viscosities up to 40 cP, some ten times better than the best behaviour obtained with either the Bruck- enstein or O’Dell circuit. However, it would not oscillate well in solutions of low viscosity and high conductivity. In terms of ionic strength, which may be related to conductivity at low and moderate values, about 10 millimolar solutions are roughly the limit of this oscillator.

As with the Bruckenstein circuit, the crystal drive-level and its ability to oscillate under given conditions depend strongly on the supply voltage, which usually has to be in the range 10-14 V; generally speaking lower supply voltage were required for successful under-liquid operation. Also with total immersion the point of best stability may be achieved as for the Bruckenstein circuit by tweaking the supply voltage. Ideas for auto-control will be discussed later. With all dry-canned crystals in ambient atmosphere the oscillator stability was as good as that of the simple well-known low-dissipation single-device Colpitts and Hartley circuits. The oscillator also has the advantage of a high output voltage, but in any case was always used buffered through a BSX20 emitter follower. Being essentially untuned, it will work for all the crystal frequencies in the range 1.7-20 MHz with only small adjustments of the 40 pF trimmers, see Fig. 5. The only disadvantages are that small AC, effects are still present and the fact that the e.s.r. cannot be estimated. A knowledge

66

68pf

insert l- 10n resistor here to measure e.s.r. in conjunction with lock-in amplifier

Fin. 6. Author’s series-mode oscillator circuit. All capacitors in nF range are disc ceramic and in pF range are silvered mica. Circuit values are for 14 MHz AT-cut crystals.

of the e.s.r. value is important in work with other types of crystal with which the author is involved, for example X-cut torsional viscosity monitoring crystals. It is believed that the ability to estimate the e.s.r. for AT-cut sensors will also be a useful future asset. Mason [14] shows that both a frequency change and a motional resistance change are important in calculating the viscoelastic parameters of immersion media for torsion crystals. AT-crystals should theoretically have a similar link with the viscoelastic moduli of their immersion media as their torsion coun- terparts, and for immersion in Newtonian fluids this case has recently been shown to be mathematically analysable [15]. When the mechanical load on the crystal surface becomes more complicated, as in the case of chemically immobilized molecular layers, the crystal-load-immersion medium regions are best treated as a series of distributed mechanical impedances [ 161.

In order to attempt to maximize data to test the above criteria, it was therefore decided to design and construct a series-mode oscillator, where one side of the crystal would be grounded (for electrochemists’ use) or close to ground potential where the e.s.r. could be estimated by phase-lock techniques.

5.2. The series circuit The finalized version of the series oscillator

is shown in Fig. 6. Essentially it consists of a three-stage tuned amplifier using BSX 20 transistors in Class A. The input is a common- base stage giving the prerequisite very low impedance for series operation and zero phase-shift. The main gain is provided by the second transistor, which operates in common emitter mode and provides 180” phase shift. Then low-impedance output is provided by an emitter-follower stage with zero phase- shift. A ferrite slug-tuned transformer then gives the necessary further phase-shift for series-mode operation (total shift in the feedback loop is 0”) and feedback is completed via the secondary of this transformer to the amplifier input and current flows through the crystal to earth. The parallel mode of the crystal is further suppressed by providing a low-value shunt resistor. This also helps to minimize phase-shift effects when the crystal is used in cases where total immersion in electrolytes and buffers may occur (see above). By lifting the earthy end of the crystal and inserting a small series sampling resistor, the crystal es-r. may be estimated. Assuming constant drive, the radio frequency potential across this resistor may be measured by means of a probe. Better still, a lock-in amplifier referenced to the output voltage, see Fig. 7,

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2-l v 1R

Fig. 7. Block diagram for crystal e.s.r. estimation.

gives a measure of the crystal series current at resonance, so that its e.s_r. becomes cal- culable.

The oscillator was designed specifically for 14.333 MHz AT-cut silver electroded crystals and although it is tuned it will usually work without modification for crystals in the range 13-15 MHz. Altering the inductors, particularly the tuned phase-shift transformer, should theoretically allow for operation with other crystal frequencies. With the former crystals it was tested with viscous loading in glycerol-water mixtures. The crystals carried on oscillating with solution viscosities as high as 40 cP, although the stability was compro- mised above 10 cP. This is thought to be due to electrode adsorption effects.

This oscillator, together with the parallel- mode oscillator described above, has recently been employed for biosensing of glucose [16, 171 and minute quantities of erythrocytes [17]. In these cases, the viscous load on the crystals is not so severe, but the initial loading due to the specialist chemisorbed layers is quite large, typically giving some 3-4 kHz downward

shift in the fundamental frequency per layer. As with all marginal oscillators, successful behaviour of the series oscillator was again very dependent on supply voltage. It requires a lower d-c. voltage than the parallel oscillator, generally very close to 7.5 V d.c. The voltage window for stable operation in any particular situation was far less than that for the parallel oscillator and a slight drawback which the user should be aware of is that this window is typically only a few tens of mV. Such an unusual feature requires some operator fa- miliarization time.

It was observed experimentally that the oscillator supply voltage had to be reduced in order to maintain stable oscillation when the crystal was totally immersed in liquid. This is not consistent with inadequate gain per se but rather with some peculiarity, as yet unex- plained, in the crystal stiction function upon immersion. An automatic circuit for series insertion into the power supply line to attempt to compensate for such effects and capable of upward or downward voltage adjustment has been tested by the author on this and several other oscillators. The schematic diagram of this system is shown in Fig. 8. By using a heterodyne system and quite a narrow filter, the circuit has the added advantage of preventing crystal operation on spurious modes. In this control system the gain of the oscillator is adjusted directly as a function of power supply voltage and can be sent in the reverse direction by interchanging the connections at the inverting and non-inverting inputs of the 741. Finer adjustment is possible by closing Si, as is required for the series- mode oscillator, see Fig. 8. Further as yet

mixer I .F amp l 9v

I I I 1 1 1

Fig. 8. Oscillator power rail control circuit. Close Sl for fine control.

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untried ideas for drift limitation are given below.

6. Control circuits and future possibilities

Two of the common problems experienced by the author have been failure of published oscillator circuits to oscillate under liquids and excess drift. Suitable control circuits to counter these problems fall into two cate- gories, namely automatic gain control (a.g.c.) and automatic frequency control (a.f.c.). The former has already been implemented here (see above) and by Thompson for discrete circuitry [7]. in principle it should be even easier to implement a.g.c. for any oscillator based around an amplifier chip having an integral a.g.c. pin. In cases of oscillators based around discrete components or logic gate families operated in the linear mode, some aspect of gain control may be brought about by a series pass element on the power supply rail, as shown above.

Automatic frequency control equipment with a long time constant to counter slow drift of the crystal frequency during biosensing as a result of temperature fluctuations and non-specific chemisorption should also be possible. A possible block diagram for such an arrangement, as yet untested, is shown in Fig. 9. Once operation of the circuit has achieved the most stable operating point for the crystal, the appropriate supply voltage could be main- tained during analyte introduction so that the relatively gross and sudden frequency changes to be expected as a result of chemical binding could be recorded. It should be noted that the action of the system proposed in Fig. 9 is dependent on frequency pulling as a result of changes in the oscillator supply voltage. If this is insufficient then additional external

Fig. 9. Block diagram of possible drift limiter/a.f.c. circuit. f, -oscillator frequency, fz = reference frequency.

variable reactance incorporating a varactor diode could be added to the crystal circuit.

The degree of intelligence awarded to future control circuits is expected to be strictly at the will of the designer. Straightforward analog designs may be employed if commercial frequency counting and recording equipment is to hand, or alternatively microprocessor- based systems could be envisaged to give a whole system-design concept. From the author’s experience with biosensing, slow frequency drift and crystal-to-crystal variation, the latter also having been reported by Mur- amatsu ef al. [18], are almost invariably a problem. Thus such control methods will be required if AT-crystal sensor systems are to be more successfully exploited in this area together with the fields of electrochemistry [S, 61 and non-equilibrium liquid interfacial chemistry [7], where the great future potential of the AT-crystal technique has also recently been cited [6].

7. Conclusions

Some successful improvements to existing oscillator designs have been achieved. Two new and potentially very useful ‘under-liquid’ oscillator designs and a simple control system have been described. To reiterate, it is hoped to publish more work on under-liquid piezo- biosensing in the near future [17]. The work described above is perceived as being useful for would-be advocates of quartz crystal sensing, particularly for scientists from non-engineering backgrounds, who have previously had to ‘pick and make do with’ their oscillator designs. Certainly in the light of the above, there are now two more from which to choose. Hopefully their choice will be narrowed in most under-liquid applications to these two alone, for according to the author’s tests they appear to surpass by far all the others tried in these gruelling conditions.

Acknowledgements

The author wishes to thank the SERC for their continued financial support during this work. Thanks also to my colleagues Professor T. J. Lewis, Dr J. P. Jones and Dr Claudius D’Silva for particular discussions on the phys-

its and chemistry of crystal biosensing. Finally, although by no means least, I thank Mr A. R. Patrick, Director of Gwynedd Commu- nications Ltd., for valuable discussions on oscillator circuitry and theory.

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G. 2. Sauerbrey, Verwendung von Schwingquarzen zur Wagung dunner Schichten und zur Mikrowagung, Phy- si&, 155 (1959) 206. Application report on IQTS-156 262.144 khz quartz temperature sensor (1987), available from IQD Ltd., Crewk- erne, Somerset, U.K. T. Nomura and M. Okuhara, Frequency shifts of piezoelectric quartz crystals immersed in organic liquids, Anal. Chin Acta, 142 (1982) 281-284. C. Hamilton and A. Gedeon, A gas analyser based on the quartz crystal adsorption technique, /. Phys. E: Sci. Instrum., 19 (1986) 271-274. S. Bruckenstein and M. Shay, Experimental aspects of the use of the quartz crystal microbalance in solution, Electrochim. Acta, 30 (1985) 1295-1300. R. Schumacher, The quartz microbalance: a novel approach to the in-situ investigation of interfacial phe- nomena at the solid/liquid junction, Angew. Chem., 29 (1990) 329-343. M. Thompson, G. K. Dhaliwal, C. L. Turner and G. S. Clabrese, The potential of the bulk wave device as a liquid phase immunosensor, IEEE Trans. Uhrason., Ferroelectr. Freq. Control, UFFC-34 (1987) 131-135. T. H. O’Dell, A low dissipation Meacham bridge Os- cillator,Electron. Wireless World, 94 (appendix A) (1988). 0. Melroy, K. Kanazawa, J. Gordon and D. Buttry, Direct determination of an underpotentially deposited monolayer at lead in gold, Langmuir, 2 (1986) 697-700. C. Barnes, Some new concepts of factors influencing the operational frequency of liquid-immersed quartz microbalances, Sensors and Actuators, to be published. Author’s unpublished work.

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L. H Grande, C. R. Geren and D. W. Paul, Detection ofgalactosyltransferaseusingchemicallymodifiedpiezo- electric quartz, Sensors andActuators, I4 (1988) 387403. Author’s unpublished data. W. P. Mason, Measurement of the viscosity and shear elasticity of liquids by means of a torsionally vibrating crystal, Trans. ASME, 69 (1947) 359-368. A. A. Tabidze and R. Kh. Kazakov, High frequency ultrasonicunitformeasuringthecomplexshearmodulus of liquids, frmer. Tekh., 1 (1983) 34-36. C. Barnes, C. D’Silva, J. P. Jones and T. J. Lewis, A concanavalin - A coated piezoelectric crystal biosensor, Sensors and Actiators, to be published. C. Barnes, C. D’Silva, J. P. Jones and T. J. Lewis, Immobilised lectin quartz crystal biosensors, submitted for presentation at Eurosensors ‘91, Rome, Italy. H. Muramatsu, J. M. Dicks. E. Tamiya and I. Karuba, Piezoelectric crystal biosensor modified with protein A for determinate of immunoglobins, Anal. Chem., 59 (1987) 2760-2763.

Biography

Christopher Barnes obtained a BSc. Honours degree in the science of materials form the University of Wales, Bangor, in 1977, an M.Sc. in electronic materials and devices in 1979 and a Ph.D. in 1982, entitled The Electronic Properties of the Metal-Dielectric Interface, with Particular Reference to Biological Materials. He has worked in several areas of interdisciplinary science, including surface tribology, charge transport in insulators and polymers, ultra- sonics and piezoelectric devices, all with a strong emphasis on measurements on biological materials in both solid and liquid phases. He is currently employed as a research fellow of the University of Wales, Bangor, engaged in studies of under-liquid biosensing employing various piezo-devices.

Development of quartz crystal oscillators for under-liquid sensing

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