Reusable and recyclable quartz crystal microbalance sensors

Reusable and recyclable quartz crystal microbalance sensors

Sensors and Actuators B 212 (2015) 196–199 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevie...

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Sensors and Actuators B 212 (2015) 196–199

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical journal homepage:

Short communication

Reusable and recyclable quartz crystal microbalance sensors Sribharani Sekar, Joanna Giermanska, Jean-Paul Chapel ∗ Centre de Recherche Paul Pascal (CRPP), UPR CNRS 8641, Université Bordeaux, 33600 Pessac, France

a r t i c l e

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Article history: Received 31 October 2014 Received in revised form 31 January 2015 Accepted 2 February 2015 Available online 14 February 2015 Keywords: QCM sensors Polystyrene thin film Ceria nanoparticles Surface functionalization Layer-by-layer growth Economical process

a b s t r a c t In this communication, we report a simple procedure to recycle quartz crystal microbalance sensors. In particular we show that a sacrificial functional PS thin film deposited on top of silica (or gold) sensor does not influence the features and reproducibility of the benchmarked PSS/PDADMAC polyelectrolyte multilayer growth. A simple rinsing of the assembly with toluene at the end of a given experiment enables the reusing of the same sensor for tenths of times or more providing a neat economical and ecological gain; an approach that hopefully will spread out. © 2015 Elsevier B.V. All rights reserved.

1. Introduction A quartz crystal microbalance (QCM) provides an interesting tool to characterize the mass and viscoelastic properties of complex thin films during their formation at the surface and once formed under perturbation in their environment. This nanogram sensitive technique utilizes acoustic waves generated by an oscillating piezoelectric single crystal plate to measure changes in the mass attached to it [1,2]. One of the unique advantages of the QCM technique is that it does not require any specific labeling of species. Early application of QCM was very much focused on gas and biological sensors. It has nowadays become a key technique widespread in many different scientific fields where the assembly of polyelectrolytes, colloids, macromolecules, proteins, nanoparticles and many others nano-objects at a liquid/solid interface is investigated [3,4]. The quartz sensors are however a quite expensive part of the QCM technique, especially if they are coated with an additional specific layers to allow adsorption or grafting of a particular molecule and/or if the system under investigation needs to probe different experimental conditions. Most of the time however physisorbed or chemisorbed nano-objects cannot be removed/desorbed/ungrafted easily at the end of an experiment [5]. The same sensor cannot be used again and a brand-new one is necessary to pursue the investigation and/or study different parameters; an approach that can be economically costly and not sustainable. For all these reasons, we have developed a simple procedure which allows reusing and recycling ad infinitum

or more pragmatically tenths of times any sensor bearing a silica (or gold) top layer. In brief, the Q-sensor is first silanized and covered with a thin and smooth layer of polystyrene (PS) appropriately functionalized before starting the planned QCM experiment. At the end of the study, the PS film dissolves easily when immersed in toluene or THF carrying away any layer built up on top, leaving behind a “clean” sensor ready to be recycled and reused again. PS has been chosen for three main reasons: (i) thin film processing is easy to handle from dilute organic solutions, (ii) formed films are glassy and do not contribute then significantly to the damping of piezoelectric sensor [6], (iii) its inherently low surface energy and poor polarizability make it a widespread polymer found in many applications as inert substrates to immobilize biological compounds like antigens or antibodies or as protection against oxidation of the silver electrodes of commercial quartz sensors [7,8]. Furthermore, PS layers are known to not swell in water [25] insuring the integrity of the sacrificial layer during the experiment and then the stability of the QCM signal. In this short communication, we describe the step by step procedure enabling the reuse of common Q-sensors. We show the innocuous behavior of the sacrificial PS coating on a particular system, i.e. the benchmarked layer-by-layer (LbL) adsorption of PSS/PDADMAC polyelectrolyte multilayer [9]. 2. Materials and methods 2.1. Materials

∗ Corresponding author. Tel.: +33 556845673. E-mail address: [email protected] (J.-P. Chapel). 0925-4005/© 2015 Elsevier B.V. All rights reserved.

Poly(styrene) [PS] Mw ∼ 250 kDa was purchased from Acros, whereas poly(diallyldimethylammonium chloride) [PDADMAC]

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Mw ∼ 100–200 kDa 20% in water, poly(4-styrene sulfonic acid) [PSS] Mw ∼ 75 kDa 18% in water and octadecyltrimethoxysilane (ODTMS) were purchased from Aldrich. The 10%/w cerium oxide nanoparticles (ceria NPs) aqueous solution (Rh ∼ 4 nm,  ∼ 6–7 g/cm3 ) was a gift from Rhodia specialty chemical (France).

2.2. Methods 2.2.1. QCM A QCM set up with dissipation, QCM-D (Q-Sense E4 instrument, AB, Sweden) with silica-coated quartz crystal sensors was used to monitor the LbL growth of the oppositely charged PSS/PDADMAC strong polyelectrolyte pair. QSX335 sensors (Q-Sense) dedicated to ellipsometric measurements have been used, where the active sensor face is covered with an opaque bottom layer of titanium, a thin layer of titania (<1 nm) and a top layer of silica (∼20 nm). The AC voltage close to the resonant frequency of the crystal (4.95 MHz) was applied to the electrodes and 6 odd overtones and energy dissipation D, were monitored at temperature of 25 ◦ C after the driving power has been switched off. Any adsorption of the mass from the solution decreases the oscillator frequency and can contribute to additional dissipation. If the adsorbed mass is forming a rigid homogeneous lossless film, the relation between frequency change Fn and the areal mass increment m is linear, as found by Sauerbrey [10] Fn 2F 2 = − √ o m n q q where Fo is the resonant frequency of the crystal; q (2.648 g/cm3 ) and q (2.947 × 1011 g/cm s2 ) are quartz density and shear modulus, respectively. Thicknesses were estimated from ellipsometry using a UVISEL spectroscopic ellipsometer (Horriba Jobin Yvon, France). The data were acquired at three angles of incidence: 65◦ , 70◦ and 75◦ in the wavelength ranging from 260 to 860 nm. Data were fitted to a multi-layer model (titanium [16]/titania [17]/silica [16]/PS [18,19]/ceria [20] (optionally)/PEM/ambient) using the DeltaPsi Horriba built-in program. Non-absorbing Cauchy model was applied to PS layer [18] and PEM, which were considered as homogeneous films. Data were acquired in air sequentially on the bare, functional PS and on the final multilayer assembly. Advancing water contact angles ( a ) were measured with a Tracker tensiometer from Teclis. AFM images were acquired in air in tapping mode with a Dimension ICON from Bruker.

2.2.2. Silanization In order to have a good adhesion with the PS layer, the silica covered sensor was cleaned with ethanol then UVO treated and finally silanized from a 2%/w ODTMS solution in toluene. The grafted layer was left for 1 h at 95 ◦ C under vacuum to remove any trace of toluene and post-cure the silane layer ( a ∼ 105◦ ). It should be noted that our approach works for gold sensors as well with the help of methylated-thiols.

2.2.3. Spin coating A 2.8%/w PS in toluene solution was then spin-coated (2450 RPM) on top of the hydrophobic silanized silica sensor giving rise to a smooth and uniform 160 nm thick PS film [11]. The layer was then post-cured at 95 ◦ C for 2 h to release any mechanical stress and enhance adhesion giving rise to a very stable PS coating under water. The hydrophobic PS covered Q-sensor was then subjected to functionalization.

Fig. 1. (a) Frequency shift (5th overtone) vs. time of ceria NPs adsorption onto a PS coated sensor (b) (1 ␮m × 1 ␮m) AFM image of the densely packed nanoceria monolayer (RMS roughness ∼0.9 nm).

2.3. Functionalization of PS In order to enable the electrostatic/hydrogen bonding adsorption of the polyelectrolytes (or any others nano-objects), charges have to be created on top of the hydrophobic and inert PS film. Therefore two simple functionalization approaches were used. 2.3.1. Nanoparticles The PS covered sensors were dipped during 2 h in a 0.1%/w ceria NPs solution containing 0.1 M NaNO3 giving rise to a densely adsorbed monolayer of positively charged NPs [12]. The sensors were then rinsed in water and dried in dry nitrogen and immediately used in QCM experiment. If needed, the initial surface charge can be reversed through the adsorption of an extra oppositely charged polyelectrolyte. Fig. 1a shows the QCM trace of the ceria NPs adsorption on a PS covered sensor. The very reproducible frequency shift of 90 Hz corresponds to an adsorbed amount of 16 mg/m2 , as calculated from Sauerbrey relation in the case of rigid nanoparticles. The calculated surface coverage of ∼0.8 is well above the random sequential adsorption value of 0.547 [13] indicating a reaction limited adsorption (through the combined action of strong attractive van der Waals and short range hydration forces) giving rise to a densely packed and hydrophilic ( r < 15◦ ) NP’s monolayer as can be seen in the AFM image of Fig. 1b. 2.3.2. UVO An alternate and widespread way to modify the PS layer uses a UVO or oxygen plasma treatment [14,15]. Both approaches introduce oxygen based functional groups at the PS surface charging it negatively when immersed in an aqueous solution (pH > 5). A 10 min UVO treatment gives rise to a hydrophilic ( a < 10◦ ) PS surface. Prior to a QCM experiment, the functionalized PS covered sensor is left few minutes into pure water to remove any soluble material (ca 10 nm of the layer as measured by ellipsometry). 2.3.3. LbL Multilayers of (PSS/PDADMAC)12 were fabricated by the sequential and alternative injection into the QCM cell a 0.1%/w PSS or PDADMAC solution in 0.1 M NaCl. After injection the solution was left in contact with the sensor for 10 min. The QCM cell was then rinsed with 0.1 M NaCl aqueous solution between each deposition step for 10 min.


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Fig. 2. Schematic representation of the whole recycling procedure to obtain reusable QCM sensors.

After a QCM experiment, the sensor was immersed in toluene in an ultrasonic bath which quickly dissolves the PS layer and consequently removes any built-up material. The sensor was then ready to be reused as depicted in Fig. 2. The recycling procedure can of course be performed on tenths of crystals sensors at the same time. 3. Results and discussion In order to show that the PS sacrificial layer does not perturb or modify a typical QCM experiment, we have compared the multilayer growth of one of the most studied polyelectrolyte’s pair [21,22] on three different sensor surfaces: the commonly used bare silica surface together with the UVO treated and nanoceria covered PS films. This oppositely charged strong polyelectrolyte pair is well known to grow linearly if the salt concentration is below 1 M [23]. In Fig. 3, the cumulative frequency shift of the 5th overtone as a function of the bilayer number is presented.


It can be seen immediately that in all three cases the PEM’s growth is linear after an incubation period slightly different for each surface. Indeed, it is well known that the initial 4–6 so-called precursor layers are greatly influenced by the interactions with the surface [24]. The build-up of a homogenous layer on silica necessitates more bilayer than for PS covered surface as seen in Fig. 3. Furthermore, it can be noticed a final adsorbed amount slightly different on silica than on PS treated surfaces. A slightly larger roughness introduced with the NPs or the UVO modification likely give rise to larger accessible areas for adsorption. Most importantly, the growth is found linear with an almost identical slope (˛) as seen in Fig. 3a suggesting the same growth mechanism beyond the precursor layers for all investigated substrates. The reproducibility of the PEs growth was also verified on the very same sensor after 10 experiments in a row as shown in Fig. 3b with bare silica and ceria-covered PS surfaces. Moreover, ellipsometric data were completely consistent with QCM results. Multilayer grown on UVO and ceria modified PS have both a thickness of ∼55 nm (520 Hz) within 1 nm although a 38 nm (420 Hz) coating is obtained for silica with thicknesses (1.4) and total frequency shift (1.25) ratios in rather good agreement. Furthermore, even though the PS layer are not prone to water swelling [25], one can ask about the integrity of the sacrificial layer during the experiment and the subsequent stability of the signal. Fig. 4 shows the genuine frequency shifts and dissipation curves of the three different substrates investigated in this work during the first hour. As one can see, the three signals are parallel and very similar before and after the first adsorption step (see arrows) with a pretty small drift [26] on both the frequency and the dissipation insuring that the properties of the PS layer is likely not changing with time. Only the different adsorption steps are measured. One last point is worth mentioning. QCM sensors are fragile systems prone to wear and aging with the consecutive deterioration of the patterns of harmonic sidebands together with the sensitivity of the


Fig. 3. Cumulative resonance frequency shift (5th overtone) as a function of the deposited PSS/PDADMAC bilayer number (a) for bare silica, ceria-covered and UVO-modified PS supports and (b) reproducibility of the LbL deposition on silica and ceria-covered PS sensors. Continuous lines are a linear fit to the data.

Fig. 4. Genuine frequency shift and dissipation as a function of time (and adsorption sequence) for the three different substrates investigated during g the first hour of the experiment. The arrows indicate the first adsorbed layer step. Both types of curves have been vertically shifted for clarity (frequency: 100 Hz, dissipation: 10 × 106 ).

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technique. Recycling has then a physical limit which can be easily evaluated at the beginning of each QCM experiment by carefully looking at the resonant bandwidths and the position of the spurious mode [26]. If many overtones are affected, the sensor is discarded and thrown away for good. We can typically reuse our sensors at least 10 times. 4. Conclusion We have shown in this work that a sacrificial functional PS layer deposited on top of any silica (or gold) QCM sensor and adequately functionalized does not influence, beyond the precursor layers, the features and reproducibility of a typical QCM experiment like the multilayer growth of oppositely charged polyelectrolytes. A simple rinsing/soaking at the end of a given experiment of the coated sensor with toluene or THF easily dissolves the PS layer carrying with it the built-up macromolecular assembly and enable the subsequent recycling and reusing of the same sensor for tenths of times or more; a virtual circle providing a neat economical and ecological gain. Acknowledgment This research was supported by the Agence Nationale de la Recherche (ANR) under the contract ANR-09-NANO-P200-36 & ANR-13-BS08-0015-01. References [1] M.V. Voinova, On mass loading and dissipation measured with acoustic wave sensors: a review, J. Sens. 2009 (2009) 13. [2] K.A. Marx, Quartz crystal microbalance: a useful tool for studying thin polymer films and complex biomolecular systems at the solution–surface interface, Biomacromolecules 4 (5) (2003) 1099–1120. [3] B. Becker, M.A. Cooper, A survey of the 2006–2009 quartz crystal microbalance biosensor literature, J. Mol. Recognit. 24 (5) (2011) 754–787. [4] A.L.J. Olsson, et al., Using the quartz crystal microbalance with dissipation monitoring to evaluate the size of nanoparticles deposited on surfaces, ACS Nano 7 (9) (2013) 7833–7843. [5] K. Gotoh, M. Tagawa, Detachment behavior of Langmuir–Blodgett films of arachidic acid from a gold surface studied by the quartz crystal microbalance method, Colloids Surf. A: Physicochem. Eng. Asp. 196 (2–3) (2002) 145–152. [6] R. Lucklum, C. Behling, P. Hauptmann, Gravimetric and non-gravimetric chemical quartz crystal resonators, Sens. Actuators B: Chem. 65 (1-3) (2000) 277–283. [7] S.P. Sakti, et al., Thick polystyrene-coated quartz crystal microbalance as a basis of a cost effective immunosensor, Sens. Actuators A: Phys. 76 (1–3) (1999) 98–102. [8] S.P. Sakti, D.J.D.H. Santjojo, Improvement of biomolecule immobilization on polystyrene surface by increasing surface roughness, J. Biosens. Bioelectron. 3 (3) (2012). [9] P. Nestler, M. Pabvogel, C.A. Helm, Influence of polymer molecular weight on the parabolic and linear growth regime of PDADMAC/PSS multilayers, Macromolecules 46 (14) (2013) 5622–5629.


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Biographies Dr. J.-P. Chapel is a senior research scientist at CNRS working at the Paul Pascal Research Institute (CRPP) within the University of Bordeaux (France). He received a Ph.D. in experimental soft matter physics from Paris University/Ecole Normale Supérieure in 1993. His current research interests are focus on macromolecules and colloids interaction in bulk and with interfaces. Dr. Joanna Giermanska received her Ph.D. in Physical Chemistry from Technical University of Wroclaw (Poland) in 1984. She is a senior research engineer at the Paul Pascal Research Institute (CRPP) within the University of Bordeaux (France). She is specialized in the processing and characterization of functional surfaces and thin polymer films. Dr. Sribharani Sekar pursued her Ph.D. in Physical Chemistry at Paul Pascal Research Institute (CRPP) within the University of Bordeaux (France). She is currently a post-doctoral fellow at the Institut Charles Sadron within the University of Strasbourg (France). Her recent research interest focuses on synthesis and orientation of anisotropic nanomaterials.