AnalytKa Chtmtca Acta, 269 (1992) 167-175 Elsevler Science Pubhshers B V , Amsterdam

167

Thickness-shear mode acoustic wave sensors and flow-injection analysis

Mengsu Yang and Michael Thompson

Department of Chemutry, Unrvers@ of Toronto, 80 St George Street, Toronto, Ontano M5S IA1 (Canada)

(Received 11th May 1992, revised manuscript received 8th July 1992)

Abstract

A feaslblhty study of the combination of the flow-mJectlon analysis (FIA) technique with the plezoelectrx acoustic wave Sensor (thickness-shear mode, TSM) IS described Flow-inJectIon systems provide a method to relate the hydrodynamic response of the sensor to the concentration-time profile of the analyte produced by the dispersion behavlour of sample zones Network analysis of the TSM sensor generates multi-dimensional mformatlon on the bulk properties of the liquid sample and surface propertles at the hqmd/sohd interface The relationship between acoustic energy transmlsslon and the mterfaclal structure, v~scoslty, density and dielectric constant of the analyte IS dlscussed The FIA-TSM system provides a rapid, reproducible detectlon method with enhanced sensltlvlty and scope

Keywords Acoustic methods, Flow mjectlon, Plezoelectrlc sensors. Acoustic wave sensor

Thickness-shear mode (TSM) plezoelectrlc acoustic wave devices have been used as a ml- crobalance m the gas phase based on the observa- tion of Sauerbrey [l] that changes m the resonant frequency of the device are proportional to mass changes on the plezoelectrlc crystal In recent tunes, Increasing attention has been pald to the operation and apphcatlon of the TSM sensors m the hquld phase [21 Studies have been made m areas mcludmg hquld chromatographlc detection [3], mununoassay [4], hquld-phase properties [5] and the electrochemical mlcrobalance 161 In ad- dltlon, several attempts have been made to pro- vlde theories for couphng of the osclllatmg sensor surface to a liquid medmm [7-111 These have been reviewed recently [21

Previous studies of TSM devices m hqulds have almost exclusively used the oscillator

Correspondence to M Thompson, Department of Chemistry, Umverslty of Toronto, 80 St George Street, Toronto, Ontano MSS 1Al (Canada)

method In this method the quartz crystal 1s part of an oscillator cn-cult It 1s connected between the output and input of the oscillator amplifier and provides posltlve feedback to cause oscllla- tlon of the clrcult The resonant frequency of the quartz crystal IS measured by a standard fre- quency counter There are some serious hmlta- tlons associated with the oscillator method as the series resonant frequency IS the sole parameter measured, the method only partially characterizes the device, the resonant frequency depends on the components of the oscdlator clrcult, and the oscillator will not function in certain situations, notably when the crystal 1s m a hquld of high v~cos~ty More recently, the network analysis method has been developed to characterize com- pletely a TSM acoustic wave device [12] This technique provides the characterlstlc parameters to more completely descnbe the behavlour of the TSM device m the hquld phase The prominent parameters obtamed from the Impedance mea- surements are the resonant frequencies, the

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168 M Yang and M Thompson /Anal Chrm Act4 269 (1992) 167-175

phases and magnitudes of the impedances and the equivalent electrical clrcult elements All the quantities can be related to physical propertles of the quartz and surrounding envn-onments through electromechamcal analysis [12,13]

In this paper the use of network analysis of the TSM quartz crystal sensor m combination with the flow-mJectlon technique 1s reported Flow-m- Jectlon analysis (FIA) is a well known and widely used technique, especially m the field of process momtormg, owing to its relative slmphclty and high precision [141 The apphcatlon of gradient techmques, the versatlhty of mamfold design and the use of FIA as a sample transport system have also made FIA an attractive tool m the field of clmlcal analysis and biotechnology [l&16] Com- bination of FIA and the TSM acoustic wave sen- sor will provide new opportumtles for detection capablilty m automated analysis The purpose of the work described here was to evaluate the feaslblhty of couphng a flow-mJectlon system to a TSM sensor and to ldentlfy the analytical fea- tures of the combmatlon

(a)

(b)

Sample \

P

Sample

EXPERIMENTAL

Reagents Glycerol (Wsher), potassium chloride (BDH),

sodium chloride (BDH) and hthmm chloride (Alfa Inorgamcs) were dissolved m doubly distilled water to prepare standard solutions Hexade- canethlol (Aldrich) was used as received All other chemicals were of analytical-reagent grade Dou- bly distilled water was used as the carrier

Apparatus A single-hne FIA manifold (Fig la) was used

111 all experiments A four-channel EVA-Pump Model 1000 perlstaltlc pump (Eppendorf) was adapted and combined with a EVA-Valve Model 2000 injector valve (Eppendorf) The PTFE tub- mg for the sample loop had an 1 d of 0 5 mm and all other tubing had an 1 d of 0 8 mm

The ~-MHZ AT-cut quartz plezoelectnc crys- tals coated with either gold or sdver electrodes were supplied by International Crystal Manufac-

Fig 1 (a) Smgle-hne mamfold for the FIA-TSM system The sample (S) IS injected mto a earner stream (C) and then passed through an au trap (A) and mwng cod (L) before reachmg the TSM acoustic wave detector (D) (b) Flow cell a = plexlglas blocks, b = screws and sprmgs leading to net- work analyser, c = gold leads, d = 0-rmgs, e = plezoelectnc crystal The two blocks are held together with a clamp The diagram IS not to scale

turmg (Oklahoma City, OK) The instrument used to characterize the quartz crystal was an HP 4195A Network/ Spectrum Analyser (Hewlett- Packard) The HP 41951A impedance test lut and HP 16092A spring clip fiiure were used to make impedance measurements directly The values of the equivalent clrcult elements of the quartz crys- tal are calculated internally by the HP 4195A from the measured data

The electrodes of the quartz crystals were cleaned m a PD3XG plasma cleaner (Harrlck) to generate a hydrophlhc surface The hydrophobic surfaces were prepared by self-assembly of hexa- decanethlol on to the gold or silver electrodes following a pubhshed procedure 1171 A Remy- Hart gomometer was used to measure the ad-

M Yang and M Thompson /Anal Chun Acta 269 (1992) 167-175

vancing contact angles of water on various sur- faces

Flow cell The design of the flow cell 1s shown m Fig lb

The two Plexiglas blocks are held together with a clamp to give a cell volume of about 10 ~1 for the liquid flow The other side of the crystal was under nitrogen m order to reduce the mterfer- ence from the atmosphere The electrodes of the crystal held m the flow cell are connected with two gold leads, which are fixed with screws on the block Two springs are connected to the screws and lead to the HP 16092A fixture with a cable

Procedures The HP 4195A network analyser was cah-

brated at a centre frequency of 9 MHz (with 120 kHz band width) The instrument scans 100 points across the frequency range m 0 5 s A user pro- gram was edited to scan the frequency range and collect the character&c parameters contmu- ously The acqulsltlon time for each set of param-

169

eters can be controlled from less than 1 s to 10 mm It was adjusted at 2 3 s per set for all the experiments unless specified otherwise

RESULTS AND DISCUSSION

Network analysts method Detailed theories to describe the behavlour of

the plezoelectrlc crystal have been derived by Cady [18] and Bottom [19] The analysis of the coupling between mechanical displacement and electrical potential m the plezoelectrlc quartz crystal results in a complex quantity as a function of the properties of quartz and frequency The complex quantity, defined as the electrical impedance (the reciprocal of admittance), 1s the ratio of the voltage across the crystal to the current flowing through the crystal The complex quantity can be interpreted as an electrical clr- cult, which responds to an applied voltage m the same way as the quartz crystal itself

(b)

Rg 2 Clrcult elements and respectwe Impedances of the equwalent electrical clrcmt of the TSM sensor (b) Typical measurements of the magmtudes and phase angles of the Impedances of the TSM sensor with one side m contact wtth (1) air, (2) water, (3) 0 1 M KCI, (4) 36% (w/w) glycerol

170 M Yang and M Thompson /Anal Chun Acta 269 (1992) 167-175

The equivalent clrcult of the quartz crystal IS shown m Fig 2a The parallel (“stat&‘) capaa- tance C, 1s the capacitance between the two parallel electrodes on the quartz surfaces The “motlonal arm” of the clrcult (resistance R,, mductance L, and capacitance C,> arises from the electromechanical motion caused by the plezoelectrlc effect m the resonant region The impedance of each arcult element 1s also given, where o = 2rf and f IS the frequency

The impedance of the equivalent clrcult, Z, 1s the impedance of the series combmatlon of R,, L, and C, m parallel with the Impedance of C,

Z=RSjX (1)

Both R and X are functions of the four cncult elements and the frequency The magnitude and phase angle of the nnpedance Z are

(ZI =(R2+X2)’ (2)

and

curves of the Impedance measurement They can be classified mto three groups based on then characterlstlc properties frequency charactens- tics, mcludmg frequencies at zero phase, series resonant frequency f, and parallel resonant fre- quency fp, impedance charactenstlcs, mcludmg maximum impedance Z,,, mmlmum unpedance Z,,, and maxunum phase angle 8,,, and equlva- lent clrcult parameters, C,, R,, L, and C, All the quantities measured from the nnpedance and phase curves and calculated from the equivalent cmxut are listed m Table 1 for a 9-MHz gold electrode quartz crystal under different surround- ing environments

FL4 - TSh4 system

6 = tan-‘( X/R) (3)

In practice, smusoldal voltages incident on and reflected from the quartz crystal are measured for a large number of frequencies m the resonant frequency range The experunental values of the magnitude and phase of impedance can be calcu- lated at each frequency from the measured volt- ages dnd currents, and characterlstlc quantities can be found from the impedance-frequency curves Typical impedance measurements of a ~-MHZ TSM quartz crystal under different condl- tlons are given m Fig 2b, where the frequency span covers the entire resonance region (funda- mental mode) Several characterlstlc quantities can be extracted from the magnitude and phase

There are two frequencies at which the phase angle of the unpedance 1s zero series resonant frequency f, 1s the low frequency of zero phase, and parallel resonant frequency fp IS the high frequency of zero phase The theoretical expres- sions for f, and fp are obtained by solving the quadratic function of frequency at zero phase (actually the effect of dissipation of energy 1s neglected)

(4)

(5)

The series resonant frequency 1s the only quantity measured by the oscillator method

Figure 3 shows FIA profiles of the responses of the series and parallel resonant frequencies for an electrolyte solution, 0 1 M KCl, and a viscous

TABLE 1

Network analyw of a thickness-shear mode bulk acoustic wave sensor under different con&Ions (2wC)

Condltlon a fs e (Z:ree)

z (kt;rn)

Rlll &Xl md (ohm) (mH)

Au 9000970 9 021083 88 81 16 88 522 73 1s 84 1204 Water 8 995 183 9011309 58 35 316 99 16 37 322 98 10 78 OlMKCl 8994977 9008301 SO 84 310 84 10 42 320 24 100s 36% (w/w) glycerol 8 993 OS2 9 007 675 30 77 574 20 8 20 627 23 9 72

a ~-MHZ gold electrode TSM device One side m contact with the hqmd, the other protected by mtrogen

2s 97 5 81 29 03 7 76 31 1s 982 32 23 8 14

M Yang and M Thompson/Anal Chm Acta 269 (1992) 167-175 171

orgamc solution, 36% (w/w) glycerol m water The basehne noise observed was slgmflcant and penodlc, especially m the case of the series reso- nant frequency This behavlour 1s attributed to the fluctuating pressure caused by the perlstaltlc pump The magnitude of the noise can be re- duced by mcorporatmg a pulse damping device m the FIA system or using a long cod The periodic fluctuation can also be smoothed by a simple Fourier transform treatment Rather than use either of the above correction methods, the ongl- nal manifold was retained m some experiments m order to compare the effect of pulsation on the different characterlstlc parameters

The 1nJectlon of the glycerol solution (dotted lines m Fig 3) results m a decrease m both the series and parallel resonant frequencies due to viscous damping of the TSM device The viscous nature of this sample (vlsconty 77 = 3 089 g ms-‘, density p = 1088 g cmm3) also causes broadening of the FIA concentration profiles Electrolyte m- Jectlon (solid lines m Fig 3) results m only a slight change m f, This result 1s not surpnsmg,

1 7

Fig 3 FIA profiles of the series and parallel resonant fre- quency responses of the TSM sensor for 0 1 M KCI (sohd Ime) and 36% (w/w) glycerol (dotted Imes) Flow-rate, 04 ml mm-‘, inJectton volume, 30 ~1, co11 length, 30 cm

Ftg 4 FIA profiles of the parallel resonant frequency re- sponses of the TSM sensor for 0 1 M KC1 (1) steady state, (2) flow-rate, 0 8 ml mm-’ (mjectton volume 30 ~1, coil length 30 cm), (3) 0 4 ml mu-’ (30 ~1, 30 cm), (4) 0 4 ml mm-’ (50 ~1, 30 cm), (5) 0 4 ml mm-’ (30 ~1, 150 cm)

as 0 1 M KC1 has almost the same vlscoety and density as the carrier water Interestingly, how- ever, the response of the parallel frequency, f,, 1s slgmficantly shifted from the values for both pure carrier and glycerol The electrolyte condition 1s certainly expected to affect the “static” capacl- tance, C,, m view of the relatlonshlp of this parameter to fp (Eqn 5) Presumably this occurs through the formation of an electrical double layer at the sensor electrode surface In summary, the resonant frequencies are sensitive to a num- ber of properties of the hqmd, but it 1s dlfflcult to distinguish the effects of these from one another A similar observation was made by Martin et al [13], who noted that when only the resonant frequency 1s monitored, it 1s impossible to dlffer- entlate changes m surface mass from liquid prop- erties

The intensity and shape of an FIA peak de- pend on a number of parameters The effects of flow-rate, sample mJectlon volume and co11 length were studied Figure 4 shows the parallel fre- quency responses of the TSM sensor on mjectlon of KC1 under different manifold conditions The f, decrease 1s plotted as a positive signal to yield a conventional FIA profile Curve 1 m Fig 4 1s a steady-state signal generated by pumping 0 1 M KC1 solution through the detector cell contmu- ously The time delay before any signal IS ob-

172

served (curves 2-5 m Fig 4) 1s the time required for the sample to travel from the injector valve to the detector Increasing the flow-rate reduces the time lag and peak broadening (curve 2), but the magnitude of the baseline noise 1s also increased owing to the perlstaltlc action of the pump A larger sample mJectlon volume does not increase the peak height greatly (curve 4), but broadens it A 30~1 inJection gives a reasonable intensity with less peak broadening (curve 3) A longer co11 results m both a longer time delay and peak broadening with little improvement m peak height (curve 5)

Acoustic energy dmpatwn The total energy of the acoustic wave quartz

device equals the maximum kmetlc energy of the quartz plate plus the maximum kmetlc energy of the surrounding media There are both internal and external losses of electrical energy m the quartz crystal Electrical energy 1s converted mto thermal energy m the quartz as a result of mter- nal frlctlon m the crystal lattice and energy radla- tlon mto the bulk material at the edges of the plate The external energy dissipation arises from both viscous damping and acoustic radiation The external energy loss depends on the bulk proper- ties of the fluid and the nature of the interaction between the fluid and the surface of the sensor

The motlonal resistance R, represents the dlsslpatlon of electrical energy m the quartz crys- tal Figure 5 shows the responses of R, under different sample mJectlons As the concentration of the glycerol increases, the vlscoslty and density of the contacting liquid increases, which results m an increase m energy dissipation, and thus m an increase of R, (solid curves l-3 m Fig 5)

The inJection of 0 1 M KC1 solution displays a different profile (solid curve 4) At high concen- tration gradients, a decrease m R, 1s observed, but at low concentration gradients, a slight m- crease m R, IS observed The response of R, on a separate mJectlon of dilute KC1 (0 01 M) only results m an increase m R, (not shown) It appears that KC1 of high concentration reduces the energy dlsslpatlon whereas that of low con- centration increases the same parameter relative to water The vlscoslty and density of KC1 solu-

M Yang and M Thompson /Anal Chm Acta 269 (1992) 167-175

14

L :

6 I

0 2 4 6 6 10 1;

~me

2

Fig 5 Varlatlon of eqmvalent cmxut resistance R, (solid curves l-4) and muumum Impedance Z,,, (dotted curves 5 and 6) (1) 69% (w/w) glycerol, (2) 56% (w/w) glycerol, (3 and 5) 36% (w/w) glycerol, (4 and 6) 0 1 M KC1 Flow-rate, 0 4 ml mu-‘, mJectlon volume, 30 ~1, cod length, 30 cm

tlons m the concentration range 0 01-O 1 M are similar to those of water A possible explanation 1s that different mterfaclal structures of the elec- trolyte at the electrode surface may affect the coupling and radiation of the acoustic wave to- wards the liquid media The behavlour of the TSM sensor response under electrolytic solutions 1s currently under mvestlgatlon

Another characterlstlc quantity, which 1s com- parable to R,, 1s the mmlmum magnitude of impedance, Z,,, This 1s measured directly from the impedance frequency curve Z,,, also re- sponds to the energy dissipation of the system At low vlscoslty, z,,, and R, have the same magm- tude However, Z,,, also depends on other equivalent circuit parameters Thus it may ac- count for the total energy fluctuation m the sys- tem caused by various energy loss mechanisms The difference between Z,,, and R, IS shown m Fig 5 In the case of glycerol mjectlon, Z,,, (dotted curve 3) had the same FIA profile as R, with lower intensity When KC1 is injected, Z,,, has a stronger signal than R,, whereas at low concentration gradients, Z,,, shows and mslgnlfl- cant reversed response compared with R,

M Yang and M Thompson /Anal Chrm Acta 269 (1992) 167-l 75 173

Statrc capantance C, At frequencies far from the resonant fre-

quency, the quartz crystal plate IS equivalent to a simple parallel-plate capacitor havmg a capaa- tance given by

C, = kq, A/e (6) where A is the area of the electrodes, e 1s the thickness of the plate, k 1s the dielectric constant of the quartz and Ed the permrttlvlty of space In an actual experimental configuration C, also m- eludes the capacitance contribution from the cell and wires

Because only one side of the TSM device 1s m contact with liquid, the conduction current be- tween the two electrodes 1s not considered How- ever, the conductwe properties of the contacting liquid will result m a change m C, Figure 6 shows the responses of C, under different sam- ple mJectlons Whereas there 1s no change for glycerol injection (curve 11, C, increases as the electrolyte solutions of LiCl, NaCl, and KC1 (curves 2-4) are inJected, probably owing to frmg- mg of the electrical field Although the concen- trations and speclftc conductances of these three solutions are the same, the differences m the peak heights and the dispersion profiles of C, changes suggest that C, also depends on other factors such as the mterfaaal liquid structure and mteractlons due to loruc adsorption

4X5! 0 1 2 3 Time4w9 5 6 7

Fig 6 Varlatlon of static eqmvalent clrcmt capacitance C, (1) 36% <w/w) glycerol, (2) 0 1 M LICI, (3) 0 1 M NaCl, (4) 0 1 M KC1 Same mamfold as m Fig 5

Fig 7 Effect of surface wettablhty of the TSM sensor on (a) the eqmvalent cmxut Inductance L, and (b) the motlonal capacitance C, using 0 1 M KC1 Sohd line, hydrophdlc surface, dotted line, hydrophobic surface Flow-rate, 04 ml mu-‘, mlectlon volume, 30 ~1, cod length, 50 cm, data acquisition time, 5 s per set

Inductance L, and capacrtance C, The effect of mterfaclal properties on the re-

sponse of the TSM device m the liquid phase has been examined by Thompson and co-workers [4,11,20,21] The motlonal Inductance L, repre- sents the mass of the quartz plate, including the coating on its surface and the effect of the liquid loading The motlonal capacitance C, represents the elastic properties of the quartz Whereas L, determines the mass loading on the quartz crystal and the energy transfer due to viscous damping, C,,, determmes the efficiency of electromecham- cal coupling between the quartz surface and con- tacting liquid

Figure 7 illustrates the responses of L, and C, on injection of KC1 with controlled conditions on the electrode surfaces of the TSM device The gold electrodes of the quartz crystal were plasma cleaned to give a hydrophilic surface (advancing contact angle of water < 109, or treated with a long-cham thlol species [hexadecanethlol, CH,-

174

(CH,),,SH] to generate a close-packed molecular configuration yleldmg a hydrophobic surface (contact angle > 110’) Although no slgmflcant difference IS observed on the response of L, (Fig 7a), the response of C, is greater for the hydrophlhc than for the hydrophobic surface (Fig 7b), suggesting a different degree of electrome- chamcal couphng On the other hand, the similar response of L, for different surfaces mdlcates that the discrepancy of frequency responses on different surfaces 1s not caused by different mass loading

Conclusions It 1s emphasized that this study was Intended

to outline the versatdlty of the network analysis method for the study of the thickness-shear mode acoustic wave sensor and to examme the feaabll- lty of the combmatlon of the flow mjectlon tech- mque with a TSM device No attempt was made to optimize the system and to relate exphcltly the characterlstlc quantities to physical propertles of the quartz crystal, contactmg liquid and the mter- facial properties at the hquld/ solid interface Systematic studies to optlmlze both the flow cell and manifold design should lead to significant improvement m detection hmlt and dynamic range and the ehmmatlon of baseline noise The net- work analysis method completely characterizes the TSM quartz crystal sensor for all condltlons of the crystal The impedance measurement pro- vides multi-dimensional mformatlon m studying the bulk propertles of the surrounding media and the mterfaclal structure The development of an electromechamcal model for the TSM acoustic wave sensor from first principles will determine the physical slgmflcance of the characterlstlc quantities

The apphcablhty of the FIA-TSM combma- tlon 1s greater than the sum of the separate apphcatlons of FIA and the TSM sensor Flow-m- Jectlon systems can provide rapld, reproducible chemistry with Its speed, slmphclty and economy The versatlhty of the TSM sensor m studymg the mass loading, liquid properties and mterfaclal parameters will allow wide applications of this technique, which 1s stall m an early developmental stage

M Yang and M Thompson /Anal Chum Acta 269 (1992) 167-175

Potential apphcatlons of this technique he m the field of contmuous monitoring of chemical and blochemlcal processes The ability of the TSM detector to measure the vlscos~ty and den- sity of lrqulds can be used for the control of industrial fermentation and downstream process- mg The lmmoblhzatlon of selective bmdmg agents on TSM surfaces, combined with the ablh- ties of sample pretreatment of FIA (flltratlon, dilution, preconcentratlon, affinity column, en- zyme reactor, etc 1, will allow the development of automatic Immunoassay and enzyme assays and the study of drug-receptor mteractlons and DNA hybrldlzatlon

The authors are grateful to the ence and Engmeermg Research Canada for support of this work

Natural SW Council of

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