product review
Quartz Crystal Microbalances Some new innovations stand alongside the standard, reliable workhorse. Judith Handley
W
hat do NASA’s Mars Rover, human serum albumin antibodies, and oily wastewater have in common? The answer is—the quartz crystal microbalance (QCM). Some of these instruments have not changed in the five years since Analytical Chemistry last reviewed them (Anal. Chem. 1996, 68, 625 A–628 A), but they are still the instrument of choice for many applications. “If one thinks of the QCM as a mass or thickness device only, then there are many competing technologies,” says K. Keiji Kanazawa of Stanford University. But, he says, “the versatility of the QCM, with its ability to be used in liquid environments as well as [gas or vacuum and] the current ability to assess the quality factor of the resonance, provides information not available using these other methods.” Analytical Chemistry’s earlier review covered the main principles behind the QCM and the characteristics of quartz crystals. In general, the standard QCM measures the mass of a material deposited on a quartz crystal surface as a linear function of a change in the oscillating crystal’s resonant frequency. The frequency is affected by the environment at the crystal’s surface, the mass and characteristics of the coating, and the properties of the solution near the electrode surface. These factors include viscosity, density, concentration, and charge density. QCMs can measure masses ranging from micrograms to fractions of a nanogram, the mass of a layer or even a partial layer of atoms.
A basic QCM includes a source of alternating current (the oscillator), a quartz crystal, two metal electrodes on opposite sides of the thin crystal wafer, and a frequency counter. Other electronic components control process conditions and data manipulation. Table 1 lists representative QCM components and ancillary equipment.
The reader is encouraged to contact the manufacturers for further information.
QCM considerations There are many choices of QCM components. Some systems are limited to manual control, while others have different levels of electronic modules, software, or interfaces with PCs. Regardless
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product review
Table 1. Representative QCM systems and ancillary equipment. Manufacturer
Elchema P.O. Box 5067 Potsdam, NY 13676 315-268-1605 www.elchema.net
Inficon Two Technology Place East Syracuse, NY 13057 315-434-1100 www.inficon.com
Institute of Physical Chemistry Polish Academy of Sciences Kasprzaka 44 01-224 Warsaw, Poland 48 22 632 32 21, X3217 malina.ichf.edu.pl/zd-2/quartz.htm
Example QCM system(s)
EQCN-700
IC/5 thin-film deposition controller for closed-loop control of either sequential or codeposition processes
Quartz crystal holder, electroIL150 film thickness monitor chemical cell with reference and auxillary electrodes as well as a controller; user's potentiostat needed
EQCN-900 quartz crystal immittance measurement system
XTM/2 thin-film deposition monitor for either deposition or etch monitoring
Resolution Mass Frequency (Hz)
0.01 ng 0.01
0.005 (IC/5); 0.1 (XTM/2)
Applications
Adsorption, electroplating, corrosion, biosensors, antigen–antibody interactions, DNA studies, ion ingress, polymer studies, viscoelastic materials, intercalation
Accessories
Quartz crystals, quartz crystal senRotacell RTC-100 cell system, 3.5- sor heads, vacuum feedthroughs to 100-mL EQCN cells, flow and demountable cells, DAQ-616SC real-time data acquisition, PS205B potentiostat/galvanostat
Crystals Resonance frequency (MHz) Surface shape Crystal diameter (mm) Electrode material(s) Surface finish(es)
10
89.3 ng/cm2
Batteries, corrosion, deposition General vacuum deposition; EM and dissolution, plating, etching, sample preparation; thermal, elecadsorption, ion dynamics, elec- tron beam, and sputter deposition tropolymerization, ion exchange, sensors and biosensors, HPLC and FIA detection Dip-type crystal holder and com- Range of crystal holders for high plete electrochemical cell, self- vacuum and ultrahigh vacuum contained flow-through quartz holder, radial thin-layer flow, detection volume ±1 µL.
5 or 6 Plano-convex
5 or 10 Plano-convex
6 Plano-convex
14
14 and 8
14
Au, Ag Unpolished
Au, Pt, Ti, Ag, Ni, other Polished and unpolished
Au, Ag, alloy
Full line of thin-film products for vacuum deposition applications; all thin-film monitors and controllers use patented measurement system for highest measurement resolution possible
No potentiostat of special design required (nongrounded working electrode); RS-232 control/data acquisition
Deposition rate display, eight-material memory, RS-232 interface, automatic thickness termination
14 Ag, Al, Au, Co, Cr, Cu, blank Polished and unpolished
Special features 1 ms response; series resonance; low noise; grounded/floating high precision f/V: 0.0012%; outputs: f, Df, V; QCI: admittance and phase
of the size of the company or the extent of its product line, choosing a QCM is a matter of finding the right match for the analytical objective and sample conditions, say the experts. Depending on the company, QCM is a general term loosely applied to different components: the sensor; the power source for generating the oscillation frequency; or a set of components, such as the crystal, electrodes, and electronics that convert frequency change into mass. Some companies use variations of the 226 A
Vacuum deposition
0.35 ng and 0.03 ng 0.1
Intellemetrics, Ltd. 35 Cable Depot Rd. Riverside Industrial Estate Clydebank, Scotland G81 1UY, United Kingdom 44 141 952 0087 www.intellemetrics.com
term for different applications: cryogenic (CQCM) for an instrument that functions below the boiling point of liquid nitrogen or EQCM for a device used in electrochemical studies. An electrochemical nanobalance is an EQCN. With thermal control, often by a heating/cooling Peltier heat exchanger, the term is TQCM. QCM monitors usually display frequency or rate and thickness. Controller functions vary with the instrument. They handle one or more sensors, each with
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single or multiple crystals, and adjust one or more power sources to maintain a constant rate and uniformity of surface coating. Some controllers close shutters on sensors to stop processes at predetermined levels and also extend the crystal’s life. If a crystal fails, the controller closes the shutter of the failed crystal and automatically switches to a new crystal. Some controllers keep track of a single film layer, whereas others track hundreds of layers. Other controller functions are graphing data and controlling
product review
Table 1. Representative QCM systems and ancillary equipment (continued). Manufacturer
Maxtek 11980 Telegraph Rd., Suite 104 Sante Fe Springs, CA 90670 562-906-1515 www.maxtekinc.com
PerkinElmer Instruments, Inc. Attn: Princeton Applied Research 801 S. Illinois Ave. Oak Ridge, TN 37831 865-481-2442 www.par-online.com
QCM Research 2825 Laguna Canyon Rd. P.O. Box 277 Laguna Beach, CA 92652 949-497-5748 www.qcmresearch.com
Q-Sense 1000 Quail St., Suite 230 Newport Beach, CA 92660 949-250-0273 www.q-sense.com
Example QCM system(s)
PM-710 and TPS-550
QCA-917 quartz crystal analyzer
Mark 20 TQCM sensor Lab controller model 2000
Q-Sense D300 including QE 301 drive electronics for simultaneous multifrequency (1–42 MHz) measurements, QAFC 301 chamber for batch or flow measurements, and QSoft 301 acquisition software
PLO-10 and CHT-100
Resolution Mass Frequency (Hz)
0.375 ng/cm2 0.03 (6-MHz crystal)
Mark 18 CQCM FEU flight controller
1 ng/cm2 0.1
1 ng/cm2 0.01
0.00884–2.45 ng ~0.05
Applications
Electrochemistry, polymers, bio- Electrochemical deposition, elec- Contamination control on spacelogicals, gas sensing, corrosion, troplating, adsorption, biosensor craft and in laboratories; outgas electrolytic/electroless plating development, batteries testing; TML, CVCM, QTGA, ASTM E595 and E1559, HD, and semiconductors
Accessories
Flow cell, data acquisition software
Sherbrooke cell, PowerSuite electrochemistry software
Cables, hosts, software
Spin coater holder, crystal cleaning holder
5, 6, and 9 Plano-convex, plano-plano
9
3, 5, 10, 15, 25, 50 (168 soon) Plano-plano, plano-convex
5
12.5, 14, 25.4
8, square
6.3, 12.7
14
Au, Ag, Al, Pt Polished and unpolished
Au, Pt Unpolished
Au, Ag Polished and unpolished
SiO2, polystyrene, Au, Ti Optically polished
Crystals Resonance frequency (MHz) Surface shape Crystal diameter (mm) Electrode material(s) Surface finish(es) Special features
Capacitance compensation, Nongrounded crystal functions crystal resistance output; crystal with any potentiostat; resonant holders: well, dip, or flow cells admittance also measured of Teflon or Kynar
water cooling tubes and electrical circuits to maintain the temperature of the crystal environment. Some other factors to consider when purchasing a QCM are the cut, size, shape, oscillation frequency, and surface texture of crystals; the size and volume capacity of liquid cells; and the possible use of a reference crystal along with the sensor crystal. Many experts agree that thinner quartz crystals have greater sensitivity because they resonate at higher frequencies, but they are also quite fragile. Crystals can be ground so that they are thin in the center and thick on the edges to make them stronger for handling and placing in a crystal holder. According to Michael Ward of the Univer-
Adsorption, hydration, crosslinking and phase transitions of macromolecules such as proteins, lipids, and polyelectrolytes
Working on submersible models; QCM-D (simultaneous multifreradiation-insensitive models also quency and multidissipation available measurements); data analysis software enables extraction of film density, thickness, viscosity, and elasticity
sity of Minnesota, most crystals have a 0.25- to 1.0-in. diameter and resonate between 5 and 30 MHz. Most QCMs use AT cut crystals, which Richard Cernosek of Auburn University says are a “temperaturecompensated cut for the thickness shear mode at room temperature.” He says that each temperature-compensated cut is for only one temperature. Small variations in the temperature or the angle of the cut can cause small variations in the measured frequency. Cernosek says that most people choose a crystal cut for temperatures within ±25 °C of the working temperature to make these effects negligible. But he warns that one must be wary of determining what is negligible,
because a frequency shift of even a few hertz can be important in highprecision measurements. He says that he either controls the temperature or measures the temperature and compensates for it. Cernosek says that controlling the temperature of the crystal in liquid cells is not easy because of heat conduction from the liquid to the crystal. One solution to this problem, he says, is to build a large structure around the QCM and allow the temperature to equilibrate after heating or cooling the structure; alternatively, a thermocouple or other heat-measuring device near the QCM will monitor the temperature so that a correction factor can be implemented.
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product review
Table 1. Representative QCM systems and ancillary equipment (continued). Manufacturer
Sigma Instruments 1318 Duff Dr. Fort Collins, CO 80524 970-416-9660 www.sig-inst.com
Universal Sensors, Inc. 5258 Veterans Blvd., Suite D Metairie, LA 70006 504-885-8443 intel.ucc.ie/sensors/universal/
SQM-160 rate/thickness monitor
PZ-1001 immunobiosensor
SID-142 codeposition controller
PZ-105 gas-phase detector
0.32 ng/cm2 (2-s measurement) 0.025
2 ng 1
Applications
Deposition rate/thickness measurement and control in a vacuum system
PZ-1001 for direct, real-time monitoring of biomolecular reactions in liquid or gas phase; PZ-105 for measuring gas samples or dry crystals
Accessories
Full line of crystal sensors and vacuum Cell in acrylic or PEEK for flowing (70feedthroughs for use in high-vacuum µL chamber) or static (up to 1 mL) liquid systems samples; software for real-time graphical display of crystal response
Example QCM system(s) Resolution Mass Frequency (Hz)
Crystals Resonance frequency (MHz) Surface shape Crystal diameter (mm) Electrode material(s) Surface finish(es) Special features
6 Plano-convex
10 Flat disc
14
14
Au, Ag Unpolished
Au Unpolished
Up to eight sensors can be measured at once; up to four sources can be controlled at once with the SID-142
A. Robert Hillman of the University of Leicester (U.K.) points out that temperature affects the density and viscosity of liquids and the chemistry of processes such as kinetics, and these variations are reflected in the frequencies measured by a QCM. “I expect that chemically oriented factors show a larger temperature dependence [than the temperature change of the crystal],” he says. Experts say that another factor to be aware of with liquid cells is longitudinal or compression waves from out-of-plane vibrations of the quartz crystal. Cernosek says that these waves do not lose energy as easily as shear waves, so they can reflect back from distant surfaces and interfere with the frequency measurement. To keep this from happening, he says to orient the cell at an angle to the surface of the crystal or roughen the surface to diffuse any reflection. 228 A
Build or buy John Hildebrand of Maxtek notes that “there’s a lot of varied opinion out there as to what gives you the best measurement. That is why there’s quite a few people making QCMs commercially, and there’s also quite a few people making them on their own.” This statement resonates with Ward, who says that he “recommends that people build their own.” Although Ward says that commercial instruments can be reliable, he stresses that whether users build or buy, they should know the principles and be aware of conditions that affect results. For example, he suspects that viscoelastic properties of liquids or deposited films may have contributed part of the frequency signal reported in some early QCM papers. Rob Roberts of PerkinElmer expresses a different opinion. “Purchasing a com-
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mercial QCM is not only a convenience, but it offers the added advantages of access to service and technical support from the supplier,” he says.
What’s new? One type of frequency distortion in a positive-feedback oscillator, says Kanazawa, who has worked with Maxtek, arises from the “static” arm of the resonator because it is independent of the motion of the quartz and deposited film. He has helped develop a compensated phase-lock oscillator that measures the equivalent resistance of the resonator and also offsets distortions in frequency caused by static capacitance. The result, according to Hildebrand, is that frequency and resistance measurements give information about viscosity changes in polymer coatings or any other substances at the crystal surface. Hildebrand also says that the measurement resolution of instruments has improved with higher-speed circuitry that detects smaller mass changes. Kanazawa says that another innovation is an instrument that simultaneously measures frequency and dissipation, which is useful for examining viscoelastic properties. This method takes advantage of the fact that a viscoelastic-coated crystal exhibits high dissipation, unlike a crystal with a rigid coating. After the viscoelastic-coated crystal is brought to a constant oscillating frequency and the power is cut off, the oscillation amplitude decreases quickly (high dissipation). Kanazawa says that the device “records the ensuing decay of the current. From the decay curve, both the resonant frequency and the decay time can be determined. The decay time is related to the resonator losses and can be expressed as a dissipation, D. [This method] is also insensitive to any [frequency distortion] effects from the dielectric capacitance.” Other companies have developed small systems for use in space or for hand-held field instruments. Scott Wallace of QCM Research reports the development of QCMs that can cancel from their frequency measurements the effects of absorbing radiation from the sun.
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A little palm-reading QCMs have been increasingly used to follow biological processes, including mechanisms and kinetic studies, and to investigate the mechanical properties of film coatings. Cernosek sees a “push to use QCMs as biosensors.” He also notes that QCMs are now used for monitoring fluids and deposition at high temperatures, either with or without temperature compensation. But he says that an up-and-coming area of research is new materials being developed for use at temperatures “high enough to destroy the piezoelectric character of quartz.” Maria Hepel of the State University of New York–Potsdam thinks that the QCM’s biggest impact will be “on studies of biologically significant systems, such as transport through lipid bilayer membranes, drug interactions and drug de-
QCMs can measure masses ranging from micrograms to fractions of a nanogram, the mass of a layer or a partial layer of atoms. livery systems, and biotechnology with DNA and antigen–antibody interactions.” Kanazawa sees growing interest in interfacing the QCM to electrolytic solutions; exploring coatings for chemical specificity; and making QCMs part of hybrid systems, possibly together with scanning tunneling microscopy or surface plasmon resonance. He also says that there is “an exciting amount of activity in developing mathematical models . . . to reflect properties of the film and/or liquid interface that will aid the interpretation of data.” Kanazawa adds that the “means for
acquiring undistorted data is now available in several forms. But the ability to go directly from measurements to film properties would be a great step forward for the QCM.” Judith Handley is an assistant editor of Analytical Chemistry.
Upcoming product reviews July 1: Proteomic systems August 1: Automated sample preparation for MALDI September 1: Capillary electrophoresis
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