# QCM Sensors

> Source page: https://openqcm.com/qcm-sensors
> Markdown version for AI agents and readers of plain text. Site guide: https://openqcm.com/llms.txt · Everything in one file: https://openqcm.com/llms-full.txt

openQCM quartz sensors are 14 mm AT-cut quartz crystals at 5 and 10 MHz, with gold, platinum, stainless steel (AISI 316 and 304) or silicon dioxide as the sensing surface, in 12 / 5 mm and symmetric 6 / 6 mm electrode layouts. A quartz crystal microbalance reports what happens in the first few hundred nanometres above its electrode: the surface is the experiment.

### Anatomy of a sensor

Every sensor is an AT-cut quartz disc, 14 mm across, with a thin film electrode on each face. The front electrode is the sensing surface. It wraps around the edge of the blank, so both electrodes are contacted from the back and the sensing face stays free. The sensing film is about 200 nm thick on about 10 nm of titanium; the quartz blank is about 0.33 mm thick at 5 MHz and 0.17 mm at 10 MHz.

For a thin, rigid layer the frequency change is proportional to the added mass per unit area (Sauerbrey): Δm = −C · Δf / n, with C = 17.7 ng cm⁻² Hz⁻¹ at 5 MHz and 4.42 ng cm⁻² Hz⁻¹ at 10 MHz; n is the overtone number. In water the shear wave decays within about 250 nm at 5 MHz and 180 nm at 10 MHz.

### Reading a part number

AT10-14-12-AU-WRAP: AT-cut quartz at 10 MHz (AT5 is 5 MHz), 14 mm blank, 12 mm front electrode (12 or 6), gold surface (AU, PT, SS316, SS304 or SiO2), wrapped electrodes with single side contact.

### The nine sensors

| Part number (data sheet) | Surface | Frequency | Electrodes, front / back | Made for | Sensing film | Mass sensitivity | Operating range | Resonance resistance | Box | Buy |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| AT5-14-12-AU-WRAP | Gold | 5 MHz | 12 / 5 mm | Liquid biosensing | ~10 nm Ti / ~200 nm Au | 17.7 ng cm⁻² Hz⁻¹ | −20 to +125 °C | < 15 Ω | 10 pieces | store |
| AT10-14-12-AU-WRAP | Gold | 10 MHz | 12 / 5 mm | Liquid biosensing | ~10 nm Ti / ~200 nm Au | 4.42 ng cm⁻² Hz⁻¹ | −20 to +125 °C | < 10 Ω | 10 pieces | store |
| AT10-14-6-AU-WRAP | Gold | 10 MHz | 6 / 6 mm | Gas phase, vacuum and thin films | ~10 nm Ti / ~200 nm Au | 4.42 ng cm⁻² Hz⁻¹ | −20 to +125 °C | < 10 Ω | 10 pieces | store |
| AT5-14-12-PT-WRAP | Platinum | 5 MHz | 12 / 5 mm | Electrochemistry and liquid sensing | ~10 nm Ti / ~200 nm Pt | 17.7 ng cm⁻² Hz⁻¹ | −20 to +70 °C | < 15 Ω | 5 pieces | store |
| AT10-14-12-PT-WRAP | Platinum | 10 MHz | 12 / 5 mm | Liquid biosensing and electrochemistry | ~10 nm Ti / ~200 nm Pt | 4.42 ng cm⁻² Hz⁻¹ | −20 to +70 °C | < 10 Ω | 5 pieces | store |
| AT10-14-6-PT-WRAP | Platinum | 10 MHz | 6 / 6 mm | Electrochemistry, catalysis and gas phase | ~10 nm Ti / ~200 nm Pt | 4.42 ng cm⁻² Hz⁻¹ | −20 to +70 °C | < 10 Ω | 5 pieces | store |
| AT5-14-12-SS316-WRAP | Stainless steel 316 | 5 MHz | 12 / 5 mm | Biofouling and corrosion studies in liquid | ~10 nm Ti / ~200 nm SS316 | 17.7 ng cm⁻² Hz⁻¹ | −20 to +125 °C | < 15 Ω | 5 pieces | store |
| AT5-14-12-SS304-WRAP | Stainless steel 304 | 5 MHz | 12 / 5 mm | Fouling and cleaning studies in liquid | ~10 nm Ti / ~200 nm SS304 | 17.7 ng cm⁻² Hz⁻¹ | −20 to +70 °C | < 15 Ω | 5 pieces | store |
| AT5-14-12-SiO2-WRAP | Silicon dioxide | 5 MHz | 12 / 5 mm | Lipid bilayers, polymers and silica surface studies | ~10 nm Ti / ~200 nm Au / ~50 nm SiO₂ | 17.7 ng cm⁻² Hz⁻¹ | −20 to +125 °C | < 15 Ω | 10 pieces | store |

All sensors: AT cut at 35° 15′, plano-plano, polished, fundamental mode, wrapped electrodes with single side contact, titanium adhesion layer, frequency tolerance ± 5 kHz (± 7 kHz for SS304), storage from −40 to +125 °C. The same blank fits openQCM NEXT, Q-1 and Wi2, and holders built for 14 mm Q-Sense type sensors.

### Five surfaces

- **Gold (Au).**  The general purpose surface: thiol monolayers, biosensing, protein and polymer adsorption, electrochemistry. Chemically noble. Film: ~10 nm Ti / ~200 nm Au.
- **Platinum (Pt).**  Electrochemical QCM, catalysis and corrosive environments. A wide potential window, and a surface that can be regenerated many times. Film: ~10 nm Ti / ~200 nm Pt.
- **Stainless steel 316 (SS316).**  The steel of medical devices, implants and marine equipment: protein adsorption, biofouling, corrosion. More chloride resistant than 304. Film: ~10 nm Ti / ~200 nm SS316.
- **Stainless steel 304 (SS304).**  The steel of food, dairy and process equipment: fouling and cleaning-in-place studies. Film: ~10 nm Ti / ~200 nm SS304.
- **Silicon dioxide (SiO₂).**  A hydrophilic, glass-like surface: supported lipid bilayers, vesicles, polyelectrolytes, surfactants. Film: ~10 nm Ti / ~200 nm Au / ~50 nm SiO₂.

### 5 MHz or 10 MHz, 12 mm or 6 mm

A 10 MHz crystal has four times the mass sensitivity of a 5 MHz crystal. A 5 MHz crystal is twice as thick and easier to handle. 12 / 5 mm electrodes give a large sensing electrode for liquid cells; symmetric 6 / 6 mm electrodes are for gas phase, vacuum and thin film work.

|  | 5 MHz | 10 MHz |
| --- | --- | --- |
| Mass sensitivity (Sauerbrey, n = 1) | 17.7 ng cm⁻² Hz⁻¹ | 4.42 ng cm⁻² Hz⁻¹ |
| Nominal blank thickness | ≈ 0.33 mm | ≈ 0.17 mm |
| Decay length of the shear wave in water | about 250 nm | about 180 nm |
| Frequency shift in water near 25 °C | about +8 Hz/°C | about +23 Hz/°C |
| Overtones read by openQCM Q-1 and NEXT | up to n = 9 | up to n = 5 |

### Handling: six rules for every sensor

- Hold the sensor by the edge of the quartz, with plastic tipped tweezers and powder free nitrile gloves. Never touch or wipe the electrodes or the wrapped contact.
- Dry from the centre outwards with filtered, oil free nitrogen. A droplet left to evaporate leaves a ring of residue.
- Tighten the holder finger tight only. Clamping force shifts the resonance: three point clamping moved 10 MHz crystals by up to 3 ppm.
- After mounting, wait for the frequency to stabilize before taking the baseline.
- Do not use ultrasonic baths: cavitation is itself used as a delamination test for sub-micrometre coatings.
- Use one fixed pretreatment for a whole series of measurements: different cleaning methods give different surface states.

### Chemical compatibility, surface by surface

| Treatment | Gold | Platinum | Stainless steel | Silicon dioxide |
| --- | --- | --- | --- | --- |
| Water, ethanol, isopropanol, acetone | Safe. No measurable etching of gold, titanium, chromium or quartz. | Safe. No measurable etching of platinum, titanium or chromium. | Safe. Compatible. Use ultrapure water, free of chloride. | Safe. No etching of silica, titanium, chromium or gold. |
| UV/ozone | Safe. Up to 30 min. It leaves a thin gold oxide: 20 min in absolute ethanol reduces it. | Safe. At no more than 80 °C. | Safe. Short, 5 to 10 min, followed by a water rinse. A long exposure changes the oxide and the wetting. | Safe. Removes organics and does not etch the oxide. |
| Oxygen plasma | Safe. Low power and short: 22 W for 85 s caused no damage on openQCM sensors. | Caution. Forms platinum oxide or roughens the surface. | Caution. Forms Cr(VI) and changes oxide and wetting. Argon plasma is the safe choice on steel. | Safe. Removes organics and does not etch the oxide. |
| Chloride media: PBS, saline, cell media | Caution. Only under anodic potential: chloride then corrodes thin gold. | Caution. Under anodic polarization, traces of chloride increase platinum dissolution. | Caution. For the duration of the experiment only, then rinse. Never store the sensor in it. | Safe. Neutral buffers are the standard media for bilayers on silica. |
| Dilute NaOH or KOH | Safe. 50 mM KOH for cyclic voltammetry, within the potential limits of the data sheet. | Caution. Short contact only: alkali dissolves quartz faster at higher pH and temperature. | Safe. 0.1 to 0.25 M NaOH, up to 50 °C, up to 15 min: it removes proteins. | not rated |
| Hydrogen peroxide and its mixtures | Caution. Hot H₂O₂ etches chromium at 110 nm/min: avoid it on chromium adhesion layers. | Caution. Especially warm: it etches chromium at 110 nm/min at 50 °C. | not rated | Avoid. NH₄OH/H₂O₂ mixtures undercut titanium and chromium at the edges. |
| Hot or concentrated NaOH and KOH | Never. Etches the quartz and softens the titanium. | not rated | Caution. Above 0.25 M or above 50 °C it etches the quartz and softens the titanium. | Never. 30 % KOH at 80 °C removes 50 nm of deposited silica in a few minutes. |
| Hydrochloric acid | not rated | Never. Hot, with oxidants (Cl₂, H₂O₂, HNO₃), it dissolves platinum. | Never. Also HCl-based descalers and FeCl₃, the solution used to induce pitting in corrosion tests. | Caution. Dilute and short only: it does not etch silica, but the adhesion metal is exposed at the edges. |
| Acid piranha | Never. Titanium at 240 nm/min. It caused the largest loss of performance on openQCM sensors. | Never. Titanium at 240 nm/min: it undercuts the adhesion layer. | Avoid. Etches nickel quickly and undercuts the titanium. | Avoid. It does not etch silica or gold, but undercuts the electrode at edges and pinholes. |
| Aqua regia, also diluted | Never. Dissolves gold at 680 nm/min. | Never. Etches platinum at 3.6 nm/min at 30 °C, faster when hot. | not rated | Never. Dissolves the gold underneath through pinholes and edges. |
| HF and fluorides | Never. Titanium at 1100 nm/min, and it etches the quartz. | Never. Titanium at 1100 nm/min in 10:1 HF, and it etches the quartz. | Never. Attacks titanium and quartz. | Never. 10:1 HF removes the 50 nm of silica in about 2 min. |
| Ultrasonic bath | Avoid. Cavitation is itself used as a delamination test for sub-micrometre coatings. | Avoid. Cavitation is itself used as a delamination test for sub-micrometre coatings. | Avoid. Degrease by immersion instead of ultrasound. | Avoid. Cavitation damages polished surfaces. |
| Abrasives, polishing, wiping | Never. They remove the 200 nm film. | Never. They remove the 200 nm film. | Never. They remove the 200 nm film. | Never. They remove the film. |

Safe: no measurable effect in the conditions given. Caution: usable within the stated limits. Avoid: damages part of the sensor. Never: destroys the film, the adhesion layer or the quartz.

### Cleaning in three levels

#### Gold

Gold is noble, but a clean gold surface takes up hydrocarbons from the air within minutes and binds sulfur compounds and proteins strongly.

- **Level 1, between measurements.**  Flush with buffer, then ultrapure water. Rinse the gold face with ethanol or isopropanol, then water. For protein residues, 10 to 15 min in alkaline SDS (pH 11) at room temperature. Dry with nitrogen.
- **Level 2, before use.**  Solvent rinse, then a low power oxygen plasma (about 20 W, 60 to 90 s) or UV/ozone for 15 to 30 min, then 20 min in absolute ethanol to reduce the gold oxide. With a potentiostat: 10 cathodic sweeps in 50 mM KOH.
- **Level 3, deep cleaning.**  Thiol monolayers: reductive desorption in 0.5 M KOH, or 0.5 M NaBH₄ for 10 min. Proteins: alkaline SDS, then plasma or the electrochemical sequence. Acid piranha is the last resort: expect to lose the sensor.
- Full procedure with references: [data sheet AT5-14-12-AU-WRAP (PDF)](https://openqcm.com/downloads/sensors/AT5-14-12-AU-WRAP.pdf).

#### Platinum

Platinum withstands strong oxidation, but it dissolves slowly under potential cycling above 1.15 V, and a dissolved monolayer shows as a frequency drift.

- **Level 1, between measurements.**  Rinse with the measurement solvent or buffer, then ultrapure water; ethanol or isopropanol after organic or gas phase work. Dry with nitrogen or argon. With a potentiostat: 10 to 20 cycles between 0.05 and 1.20 V in deaerated 0.5 M H₂SO₄.
- **Level 2, before use.**  Acetone for 10 to 30 min, then isopropanol and water. Optionally UV/ozone for 10 to 20 min at no more than 80 °C. Electrochemical activation in 0.5 M H₂SO₄ or 0.1 M HClO₄ until consecutive voltammograms overlap.
- **Level 3, deep cleaning.**  Solvent soak for up to 1 h, UV/ozone two or three times with a water rinse in between, then potential cycling. Carbon monoxide, sulfur and metal adatoms each have their own anodic treatment. Never flame anneal.
- Full procedure with references: [data sheet AT5-14-12-PT-WRAP (PDF)](https://openqcm.com/downloads/sensors/AT5-14-12-PT-WRAP.pdf).

#### Stainless steel

A sputtered steel film forms a uniform passive layer. PBS carries about 4900 mg/L of chloride: rinse it out after every run and never store the sensor in it.

- **Level 1, between measurements.**  Flush with the running buffer, then ultrapure water for 5 min. Protein or soft soil: 0.1 M NaOH at 20 to 40 °C for 10 min, then water. Optionally a rinse in 0.5 to 1 % HNO₃ or 1 to 2 % citric acid.
- **Level 2, before use.**  Acetone, then isopropanol, 5 to 7 min each, by immersion. 0.1 M NaOH or a chloride-free alkaline detergent for 10 to 15 min. Then argon plasma for a few minutes at low power, or UV/ozone for 5 to 10 min. Rinse with water before mounting.
- **Level 3, deep cleaning.**  Proteins and biofilms: enzymatic detergent at 40 to 50 °C, then 0.25 M NaOH. Mineral scale: 1 % HNO₃ or 2 to 5 % citric acid. Citric acid re-passivation only after exposure to chloride. Never hydrochloric acid.
- Full procedure with references: [data sheet AT5-14-12-SS316-WRAP (PDF)](https://openqcm.com/downloads/sensors/AT5-14-12-SS316-WRAP.pdf).

#### Silicon dioxide

The silica layer is about 50 nm thick and deposited, not grown: fluoride and hot alkali consume it quickly. Losing 1 nm shows as about +12 Hz.

- **Level 1, between measurements.**  Flush the buffer out with ultrapure water. To remove a lipid layer: 2 % Hellmanex III at 20 to 25 °C for 10 min, or ethanol or isopropanol for 5 to 10 min. Flush with water for at least 10 min, rinse and dry with nitrogen.
- **Level 2, before use.**  Solvent rinse, then 2 % Hellmanex III at 20 to 35 °C for 10 to 30 min, without ultrasound. Rinse both faces. Activate with oxygen or air plasma for 1 to 2 min, or UV/ozone for 10 to 20 min. Use within 30 to 60 min.
- **Level 3, deep cleaning.**  Lipids: Hellmanex at 30 to 35 °C for 30 min, then a longer plasma or UV/ozone. Proteins: enzymatic detergent for 30 to 60 min. Polymers: a good solvent, then oxygen plasma. Piranha is not recommended.
- Full procedure with references: [data sheet AT5-14-12-SiO2-WRAP (PDF)](https://openqcm.com/downloads/sensors/AT5-14-12-SiO2-WRAP.pdf).

On openQCM 10 MHz gold sensors, three regeneration cycles reduced the sensor performance by 7 % with electrochemical cleaning, 16 % with oxygen plasma and 25 % with piranha (Wasilewski et al., Biosensors 12, 309, 2022). Always start with the lowest level that restores the baseline. Replace the sensor when the quartz edge is cracked or chipped, the electrode is scratched, peels or stays discoloured, the baseline is noisy, the frequency in air drifts from one cleaning to the next, or the cleanliness test of its data sheet no longer passes.

### Data sheets

- openQCM quartz sensors: specifications, cleaning and care, the catalogue of the whole range (PDF): on request, with name and email, from [the sensors page](https://openqcm.com/qcm-sensors#data-sheets)
- [AT5-14-12-AU-WRAP (PDF)](https://openqcm.com/downloads/sensors/AT5-14-12-AU-WRAP.pdf): Gold, 5 MHz, 12 / 5 mm
- [AT10-14-12-AU-WRAP (PDF)](https://openqcm.com/downloads/sensors/AT10-14-12-AU-WRAP.pdf): Gold, 10 MHz, 12 / 5 mm
- [AT10-14-6-AU-WRAP (PDF)](https://openqcm.com/downloads/sensors/AT10-14-6-AU-WRAP.pdf): Gold, 10 MHz, 6 / 6 mm
- [AT5-14-12-PT-WRAP (PDF)](https://openqcm.com/downloads/sensors/AT5-14-12-PT-WRAP.pdf): Platinum, 5 MHz, 12 / 5 mm
- [AT10-14-12-PT-WRAP (PDF)](https://openqcm.com/downloads/sensors/AT10-14-12-PT-WRAP.pdf): Platinum, 10 MHz, 12 / 5 mm
- [AT10-14-6-PT-WRAP (PDF)](https://openqcm.com/downloads/sensors/AT10-14-6-PT-WRAP.pdf): Platinum, 10 MHz, 6 / 6 mm
- [AT5-14-12-SS316-WRAP (PDF)](https://openqcm.com/downloads/sensors/AT5-14-12-SS316-WRAP.pdf): Stainless steel 316, 5 MHz, 12 / 5 mm
- [AT5-14-12-SS304-WRAP (PDF)](https://openqcm.com/downloads/sensors/AT5-14-12-SS304-WRAP.pdf): Stainless steel 304, 5 MHz, 12 / 5 mm
- [AT5-14-12-SiO2-WRAP (PDF)](https://openqcm.com/downloads/sensors/AT5-14-12-SiO2-WRAP.pdf): Silicon dioxide, 5 MHz, 12 / 5 mm

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