Monitoring the Surface Chemistry of Functionalized Nanomaterials

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Monitoring the Surface Chemistry of Functionalized Nanomaterials with a Microfluidic Electronic Tongue Flavio M Shimizu, Anielli M Pasqualeti, Fagner R Todão, Jessica Fernanda Affonso de Oliveira, Luis Carlos Silveira Vieira, Suely PC Gonçalves, Gabriela H da Silva, Mateus B Cardoso, Angelo Luiz Gobbi, Diego Stéfani Teodoro Martinez, Osvaldo Novais Oliveira, and Renato Sousa Lima ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b00056 • Publication Date (Web): 09 Feb 2018 Downloaded from http://pubs.acs.org on February 9, 2018

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Monitoring the Surface Chemistry of Functionalized Nanomaterials with a Microfluidic Electronic Tongue Flavio M. Shimizu,‡,∂ Anielli M. Pasqualeti,†,∂ Fagner R. Todão,†,∂ Jessica F. A. de Oliveira,†,§,€ Luis C. S. Vieira,† Suely P. C. Gonçalves,† Gabriela H. da Silva,†,|| Mateus B. Cardoso,†,§,€ Angelo L. Gobbi,† Diego S. T. Martinez,†,|| Osvaldo N. Oliveira, Jr.,‡ and Renato S. Lima*,†,§ †

Laboratório Nacional de Nanotecnologia, Centro Nacional de Pesquisa em Energia e Materiais, Campinas, São Paulo Instituto de F sica de São Carlos, Universidade de São Paulo, São Carlos, São Paulo -970, Brasil § Instituto de Qu mica, Universidade Estadual de Campinas, Campinas, São Paulo -970, Brasil € Laboratório Nacional de Luz S ncrotron, Centro Nacional de Pesquisa em Energia e Materiais, Campinas, São Paulo || Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, Piracicaba, São Paulo -000, Brasil ∂ These authors contributed equally to this work

‡

-970, Brasil

-970, Brasil

ABSTRACT: Advances in nanomaterials have led to a tremendous progress in different areas with the development of high performance and multifunctional platforms. However, a relevant gap remains in providing the mass-production of these nanomaterials with reproducible surfaces. Accordingly, the monitoring of such materials across their entire life cycle becomes mandatory to both industry and academy. In this paper, we use a microfluidic electronic tongue (e-tongue) as a user-friendly and cost-effective method to classify nanomaterials according to their surface chemistry. The chip relies on a new single response e-tongue with association of capacitors in parallel, which consisted of stainless steel microwires coated with SiO2, NiO2, Al2O3, and Fe2O3 thin films. Utilizing impedance spectroscopy and a multidimensional projection technique, the chip was sufficiently sensitive to distinguish silica nanoparticles and multi-walled carbon nanotubes dispersed in water in spite of the very small surface modifications induced by distinct functionalization and oxidation extents, respectively. Flow analyses were made acquiring the analytical readouts in a label-free mode. The device also allowed for multiplex monitoring in an unprecedented way to speed up the tests. Our goal is not to replace the traditional techniques of surface analysis, but rather propose the use of libraries from e-tongue data as benchmark for routine screening of modified nanomaterials in industry and academy. KEYWORDS: nanoparticles, nanotubes, surface characterization, quality control, impedance. Nanomaterials such as carbon nanotubes (CNTs), graphene, nanoparticles, and nanowires have been extensively employed in energy (storage and conversion),1 flexible electronics,2 environmental decontamination,3 medicine (drug delivery, imaging, regeneration of tissues, diagnoses, and implants),4 sensors,5 and fine chemical syntheses.6 In many of these cases, the strategy of surface functionalization with either soft or solid inorganic matters is essential to tune the properties and achieve multifunctional platforms of practical significance. In spite of the intrinsic advantages of the nanomaterials, a remaining challenge is their mass production with reproducible surfaces.2,5 For instance, in the fifth Nanotechnology Signature Initiative (NSI) launched in 2012 from U.S. National Nanotechnology Initiative (NNI), entitled Nanotechnology for Sensors and Sensors for Nanotechnology: Improving and Protecting Health, Safety, and the Environment,5 the development and commercialization of sensor nanotechnologies has been recognized as being hampered by the lack of reproducibility in the synthesis and modification of nanomaterials. In this regard, the monitoring of such materials across their entire life cycle, including processes as manufacturing, chemical functionalization, and disposal becomes mandatory to both industry and academy. Such monitoring is especially important because of the need to scale up production and impact of the surface chemistry for the success of nanomaterial-based assays. Standard methods to assess the morphology, topography, and chemical composition of the nanoscale materials include atomic force microscopy (AFM), Raman spectroscopy, X-ray crystallography, dynamic light scattering (DLS), transmission electron microscopy (TEM), Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and scanning electron microscopy

(SEM). While these methods provide accurate and precise information on the material structure and surface chemistry, they usually need high-cost instrumentation operated by qualified technicians. Furthermore, such techniques may require laborious sample treatment, time-consuming analyses, and inability to perform target detection in real time.5 These hurdles suggest the critical need for new sensor designs that should be user-friendly, fast, portable, cost-effective, and reliable to make routine experiments for monitoring modified nanomaterials. Electronic tongue (e-tongues) are promising for quality control applications in water monitoring and industries of beverages, foods, and pharmaceuticals.7−10 These platforms consist of arrays of sensing units that statistically convert the multivariate spectra in different fingerprints according to the analyzed sample. E-tongues should contain distinct sensing units (electrodes when electrochemical detectors are utilized) that are affected in distinct ways by the sample to afford diversified data. This feature is crucial to warrant a satisfactory classification of the samples. In spite of the relatively large number of e-tongues already produced, the need to recalibrate the system when electrodes are replaced as well as to accomplish subsequent surface functionalization and measurements to each one of the different sensing units are notorious drawbacks that have prevented a widespread use of such systems. These downsides undermine the simplicity and analytical frequency11 especially when it is desirable to test complex samples such as wine and coffee for which the number of electrodes may be as large as 23.12,13 To address these drawbacks, our group recently developed a new type of microfluidic e-tongue that makes use of a single measurement regardless of the number of sensing units and without the need to make

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laborious electrode modifications.11 This single signal was relative to the equivalent real admittance spectrum of an association of parallel resistors that were inserted in microfluidic devices composed of a single piece of polydimethylsiloxane (PDMS). The nonmodified film-coated resistors demonstrated satisfactory inter-electrode precision when the sensing units were replaced by new ones. Using a new type of single signal microfluidic e-tongue with improved performance, this manuscript reports, to the best of our knowledge, the first attempt to apply a userfriendly, low-cost, and multiplex setup to classify nanomaterials according to their surface chemistry. Strikingly, dispersions bearing similar physicochemical properties as revealed by SEM, DLS, FTIR, zeta potential, TGA, and XPS data could be classified with the e-tongue that was based on an association of parallel capacitors as sensing units. The monitoring of two nanomaterials is shown, namely, silica nanoparticles (SiO2NPs) and multi-walled CNTs (MWCNTs) that had different extents of functionalization and oxidation, respectively. Such nanomaterials were chosen owing to their importance in a myriad of applications. SiO2NPs can be employed, e.g., to attain selective drug delivery,14,15 whereas the oxidation of MWCNTs is essential, e.g., for subsequent dispersion and functionalization steps by creating reactive oxygenated functional groups.16−18 MWCNTs of industrial grade were used because of their wide availability. Moreover, the characterization of these materials by their manufacturers is generally incomplete for nanotoxicity assessment and biotechnological applications.19,20 Flow analyses were performed with the etongue acquiring the electric readouts for the nanomaterials dispersed in water in a label-free mode. With this application, our goal is not to replace the conventional methods of surface analysis. Instead, we present a device with complementary advantages for routine monitoring of modified nanomaterials. For instance, our platform may serve for a rapid scrutiny of nanomaterial batches, which should have already been selected on the basis of rigorous characterizations.

EXPERIMENTAL SECTION

 Chemicals. Sylgard 184 silicone, AZ

®

50XT resist, and absolute ethanol were supplied from Dow Corning (Midland, MI), Microchemicals (Ulm, Germany), and Merck (Kenilworth, NJ), respectively. MWCNTs were acquired from CNT Co. (Incheon, South Korea) and 1-[3(trimethoxysilyl)propyl]urea (TMSPU), ammonium hydroxide (NH4OH), and tetraethyl orthosilicate (TEOS) were provided by Sigma-Aldrich (St Louis, MO). The solutions were prepared in distilled water (Milli-Q, Millipore Corp., Bedford, MA), obtained with resistivity of MΩ cm. Microfabrication. The fabrication of the e-tongue illustrated in Figure 1A is similar to a previous one by our group. 29 Microfluidic devices of a single piece of PDMS featured four pairs of parallel channels on both below and above a perpendicular microchannel where the samples were pumped. Thin film-coated microwires were introduced in each one of the parallel channels acting as sensing units. The chips were produced using a bondless, low-cost, and cleanroom-free method of sequential steps of polymerization and scaffold removal (PSR).11,21,22 Briefly, a channel scaffold is shaped and, then

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PDMS is poured covering the scaffold altogether. The prototyping is completed after curing the PDMS and removing the scaffold. Stainless steel microwires defined the channels of sample and electrode. The sample channels were obtained using microwires with diameters of . , 7 . , or . µm, whereas the electrode channels were prepared from microwires with 7 . µm. Unlike the previous works where the scaffolds of electrode were inserted only below the sample scaffold, herein the microwires that defined the electrode channels were placed on both below and above the sample channel scaffold.11,21 The PDMS chip exhibited contact at the junctions of the channels that established the sensing areas, i.e., the areas of the capacitor electrodes. To prepare PDMS, monomer and its curing agent were mixed at 10:1 w/w ratio and then degassed under vacuum for 20 min. The photoresist AZ® 50XT was deposited on the microwire that defined the sample channel by centrifugation at 2000 rpm for 30 s and then dried at room temperature for 20 min. This resist was intended to avoid the filling of the scaffold junctions by PDMS to warrant an adequate contact between the sample and electrodes. The PDMS cure was conducted at ºC for 2 h. Finally, the device was hard baked at 2 °C for 2 min to complete the PDMS crosslinking reactions. Images of the channels were recorded through FE-SEM (FEI Inspect F50, Hillsboro, OR) and stereoscopy (Leica M125, Wetzlar, Germany). The latter images were valuable to measure the diameters of the channels (n = 5) using the software LAS Core V3.8. Electrodes. All the electrodes consisted of 304 stainless steel microwire (Treficap, São Paulo, Brazil) with 700-µm diameter. The microwire pairs (capacitor) were coated with distinct oxide films, namely, SiO2, NiO2, Al2O3, and Fe2O3, aiming to diversify the capacitance data. Such 800 nm thick films were deposited with the electron beam technique (Oerlikon Leybold Vacuum, UNIVEX 300, Cologne, Germany). In addition, pairs of stainless steel microwires coated with four metals (Au, Pt, Ni, and Cr) and then with SiO2 film were also tested as sensing units for classification tasks. In this case, the diversification in the e-tongue data was associated with the distinct metals rather than the oxide dielectrics. The coated electrodes were short-circuited to get an association of capacitors in parallel. Images of the microwires were obtained in a Bruker Dimension FastScan AFM microscope (Bruker Corp., Harvard, MA) with the TappingTM mode at 1.0 Hz in 10 × 10 µm2. The data were processed by the Gwyddion software. FESEM images of the microwires were also achieved in a Zeiss Sigma microscope (Jena, Germany) operating at 5 kV with surface chemical analyses from energy dispersive spectroscopy (EDS). Synthesis and surface modification of the nanomaterials. A modified-Stöber method was used to synthesize the nanoparticles as described in the Supporting Information.23 The formed NPs were modified with different TEOS:TMSPU molar ratios, namely, 1.0:0.5, 1.0:1.0, 1.0:1.5, and 1.0:2.0, whereas the bare SiO2NPs were synthesized following the same procedure, but in the absence of TMSPU. In relation to the MWCNTs, distinct samples were produced altering the oxidation time as 6, 12, 24, and 48 h following an experimental procedure described in the Supporting Information. Characterization of the nanomaterials. Particle hydrody-

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namic diameters of the NPs were reached by DLS in a Malvern Zetasizer (Nano Series, Malvern Instruments, Malvern, United Kingdom) with He-Ne laser (633 nm) and backscattering angle of 7 º. SEM was used to investigate the size and morphology of the SiO2NPs and MWCNTs in a FEI Inspect F50 (Waltham, MA) and FEI Quanta 650 FEG microscopes, respectively, at 5 kV. Suspensions of the nanomaterials ( µg mL−1) in ethanol

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Figure 1. Fabrication, channels, and electrodes. Scaffold and addition of PDMS to construct the chip (A), final chip with electrodes insulated by different oxide films and images of the channels obtained by FE-SEM and stereoscopy (B), photos of the microwires placed below and above the sample channel that was filled with red dye (C), and microwires inserted in two chips for multiplex classifications (D). In (B), FE-SEM depicts the cross section of the sample channel used in the analyses for only one sample (C2), whereas stereoscopy (on top) illustrates a lateral view of this channel with the cross section of the two channels (filled with red dye) where electrodes are inserted above (EA) and below (EB) the sample. In this image, the contact between the sample and electrode channel can be visualized. This region can also be noted in stereoscopy images (on bottom) of the microfluidic channels used for the multiplex tests (C1 and C3). The scale bars in (B) mean 10 mm (chip) and 2 µm (FE-SEM). While the images in (A,C) present five pairs of electrode, we used only four pairs in this work. In two photos of (C), the direction of the flow as well as EA and EB are highlighted.

were sonicated for 10 min, dropped on silicon wafers, dried at room temperature, and then deposited on copper grids for analysis. Zeta potential and TGA analyses were also performed for both materials. The zeta potential assays were made in triplicate (n = 3) at room temperature in a Malvern Zetasizer Nano ZS90 equipment. First, TGA was used to calculate the functionalization percentage of the nanoparticles by determining the relative mass loss of organic species anchored on the material surfaces. Then, similar analyses were intended to evaluate the oxidation extents of the MWCNTs by monitoring the relative amount of oxygenated groups on nanotube surfaces. The tests (n = 3) were made under synthetic air at 100 mL min− and of °C min− in a NETZSCH 449 F3 Jupiter (Ahlden, Germany) for nanoparticles (25 to 9 °C) and PerkinElmer Thermogravimetric Analyzer Pyris 1 °C). (Norwalk, CT) for nanotubes (2 to The surface modification extents of the nanomaterials were further ascertained by FTIR (SiO2NPs) and XPS (MWCNTs). The FTIR spectra were reached in a PerkinElmer Spectrum Two equipment using pellets of KBr at ambient conditions and transmission mode. The pellets were subjected to 32 scans with resolution of 4 cm− . The XPS tests were made in a Thermo Fisher Scientific K-Alpha (Waltham, MA) spectrometer with nonmonochromatic Al Kα radiation and flood-gun to avoid any surface charging. The pressure in the chamber was approximately 10−7 Pa and the Shirley method was utilized for background subtraction. Exploratory spectra were attained in three areas per sample with a spatial resolution of µm and energy of 200.0 eV. Analyses. Impedance spectroscopy measurements were per-

formed in an AUTOLAB Metrohm AG PGSTAT302N potentiostat/galvanostat (Herisau, Switzerland) applying voltage of 50 mV ac, integration time of 2 s, and frequencies from 1 to 106 Hz at room temperature. Two electrical connectors were utilized to short circuit the counter and reference plugs so get the association of parallel capacitors. The liquids were pumped by syringe-pumps (New Era Pump Systems, NE8000, Farmingdale, NY) and the samples were dispersed in water ultrasound bath (Branson Ultrasonics, 1210, Connecticut, NE) during 20 (1000.0 ppm SiO2NPs) and 10 min (500.0 ppm MWCNTs) before the tests. Multivariate processing. Multivariate data were processed using the whole spectra of real capacitance as a function of frequency that were normalized and dimensionally reduced using Euclidean distances as metric unit toward dissimilarity distance. The Fastmap approach was applied through the software PEx-Sensors that implements a set of techniques for information visualization.24 More specifically, we employed the nonlinear projection technique of interactive document map (IDMAP). The classification quality was quantified utilizing the silhouette coefficient (S). All of the capacitance tests were made in triplicate (n = 3) and confidence intervals were calculated for α of 0.05.

RESULTS AND DISCUSSION

 Single response e-tongue with capacitors in parallel. The new single response e-tongue based on capacitors shows several advantages compared with the previous version with resistors.11 Utilizing an association of four parallel capacitors,

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this e-tongue was more sensitive leading to more efficient classifications than those acquired with the resistors, as it will be discussed next. A circuit diagram of the association of capacitors in the e-tongue is illustrated in the Supporting Information. Each capacitor was coated with specific dielectric or metal films to create different charging abilities and, then diversify the spectra of capacitance (single signals of the etongue). Such setup of contactless electrodes (metal insulated by dielectric) is inedited for an e-tongue and inhibits limitations such as bubble formation and electrode contamination.21,25 Furthermore, the e-tongue was able to make multiplex classifications (two or more samples simultaneously). This feature speeds up the analysis in an unprecedented manner for applications with e-tongues. The use of microfluidics should also be highlighted since it decreases the consumption of chemicals and allows for automation, thus improving the analytical frequency and precision.26,27 Images of the microchip and channels are displayed in Figure 1B,C. The distance between the pairs of electrodes was 1.3 mm and the dimension of the chip was 55.0 mm × 19.0 mm × 15.0 mm. In this microfluidic chip, the surface activity of all the electrodes can be renewed after surface passivation by simply pulling the oxide-coated microwires across the channel with solution.21 The microwires coated with the thin films of oxide were short-circuited to achieve an association of capacitors in parallel as shown in Figure 1D. The sample channels used for the multiplex assays presented global diameters of 569.8 ± 0.8 µm (C1), 768.2 ± 0.2 µm (C2), and 855.3 ± 0.1 µm (C3), whereas the experiments for only one sample were performed in C2. All the channels showed circular cross section and well-defined edges according to stereoscopy and microscopy images in Figure 1B, which also display the region of contact between the sample and electrodes. FE-SEM images of the microwires and their respective size distributions are shown in the Supporting Information. The oxide-coated surfaces had high purity as revealed by EDS, while the size distributions were more heterogeneous for SiO2 and Al2O3. The average diameters of the grains on oxides were 40.5 (SiO2), 31.1 (NiO2), 135.1 (Al2O3), and 60.6 nm (Fe2O3), whereas their root mean square roughnesses (Rq) by AFM were 26.2 (SiO2), 140.0 (NiO2), 83.1 (Al2O3), and 30.4 nm (Fe2O3). The bare microwire had soft and homogeneous surfaces with Rq of only 17.8 nm. 3D AFM surface topographies of all the filmcoated microwires are displayed in the Supporting Information. Results obtained by our group (not shown) demonstrated that only the regions where the capacitors are formed (portions of the insulated microwires in contact with the liquids) impact the capacitances, as expected. Similarly to a circuit with capacitors in parallel, the rest of the microwires simply acts as conductor electric wire for current conduction. With regard to the fabrication of the electrodes, the reproducibility of capacitive and faradaic data when manually renewing the three electrodes of an electrochemical cell11 and testing different electrodes of an e-tongue,21 respectively, was already proven satisfactory. It indicates that the film deposition on the microwires is uniform. We have chosen to apply the e-tongue with four sensing units to achieve a robust and minimalist e-tongue as discussed in the Supporting Information. With regard to the principle of

detection of the e-tongue, both the oxide films and samples acted as dielectrics of the capacitors. Thereby, the samples are expected to modify the capacitance spectra depending on their dielectric constants due to the mechanisms of dipolar polarization or ionic conduction when the material presents dipoles induced by an external electric field and mobile charge carriers, respectively.28 In both these cases, the electric field created by the polarization-assisted current increases with the material dielectric constant. This phenomenon decreases the electric field among the capacitor plates with a resulting enhancement in the charging current and, then in the capacitance.29 Characterization of the functionalized SiO2NPs. Prior to demonstrating the ability of the e-tongue to classify the nanomaterials, we present a detailed surface characterization of these structures that will highlight the relevance of encountering a simple method to monitor surface chemistry. The SiO2NPs were synthesized with a modified-Stöber method as in the scheme in Figure 2A. SiO2NPs with -OH groups-rich surface were formed through reactions between ethanol and TEOS in the presence of NH4OH. Next, the silica nanoparticles were functionalized with TMSPU (organic molecule model anchored on NPs) in distinct molar ratios of TEOS:TMSPU. Only slight differences between the SiO2NPs were observed in Figure 2B-F. The nanoparticles showed similar average diameters (roughly 60.0 nm) as probed through SEM (an image of the bare nanoparticles is displayed in the Supporting Information) and number-based DLS method. All the samples were stable colloids so that sedimentation was only noted after long time. Indeed, the formation of aggregates was not verified in the capacitance assays. All the SiO2NPs had negative zeta potentials because of the -OH groups on silica surfaces.15 While the bare SiO2NPs had a potential around −2 mV, the TMSPU-coated SiO2NPs presented potentials between − and − mV. These differences between the functionalized NPs might be related to a partial aggregation of the samples that reduces the negative zeta potential as described in the literature.30 The success of the functionalization was ascertained by FTIR and TGA. For the bare particles, the FTIR spectra had characteristic transmittance bands at , , , and 9 cm−1 that were assigned to -OH stretch,31−33 angular deformations of adsorbed water on the silica surface,34 asymmetric stretching Si-O-Si,31,32,35 and symmetric stretching Si-OH, respectively (Supporting Information). The position and intensity of the bands are consistent with the literature. As expected, only slight changes were noted in the spectra of the modified SiO2NPs. Bands associated with the functionalization steps were not identified likely because of spectral overlapping from SiO2NP surfaces since the TMSPU concentration is much lower than the silica amount. In contrast, the functionalization efficiency was successfully studied by TGA. The percentages of organic compounds on SiO2NP surfaces ranged from 3.0 to 6.2%, raising with the TMSPU analytical concentrations used for functionalization, as expected. Classification of the functionalized SiO2NPs. In spite of the small changes in the dimensions and functionalization percentages of the SiO2NPs as indicated above, our e-tongue was able to classify all the samples. Each analysis lasted less than 2 min and the data processing had low computational cost. The multivariate data were processed using the whole

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spectra of real capacitance as a function of the frequency. The nonlinear projection method IDMAP was used because it led to best classifications when compared with principal component analysis (PCA) and Sammon's mapping (Supporting Information). The quality of classification was quantified by S as aforesaid. Their values vary from − . to . .36 Strong distinctions present S between 0.71 and 1.00. Below 0.71, no clear distinctions among the samples can be established. 37 Figure 3A,B presents the spectra of capacitance and IDMAP plot of the bare and functionalized SiO2NPs using the e-tongue of capacitors insulated by distinct oxide films. Analyses for dis-

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Figure 2. Characterization data of the functionalized SiO2NPs. Schematic diagram of the synthesis and functionalization of the nanoparticles (A), images and size distributions attained by SEM (B), data of number-based DLS (C), zeta potential (D), and TGA (E), and percentage of organic species on SiO2NP surfaces by TGA (F). In (B), the scale bars mean 250 nm. The nanoparticles were functionalized with different molar ratios of TEOS:TMSPU, namely, 1.0:0.5 (TMSPU (0.5)), 1.0:1.0 (TMSPU (1.0)), 1.0:1.5 (TMSPU (1.5)), and 1.0:2.0 (TMSPU (2.0)). The SiO2NP definitions in (C) are also valid for (E). A, B, C, D, and E in (D,F) mean SiO2NPs, TMSPU (0.5), TMSPU (1.0), TMSPU (1.5), and TMSPU (2.0), respectively. In (F), FP means functionalization percentage.

tilled water (control sample) were also realized after each replicate of the SiO2NPs to assess the possible contamination of the contactless electrodes of the device by SiO2NPs. The alterations in real capacitances happened at low and medium frequencies that are related to ionic and dipolar relaxations of the samples, respectively. The IDMAP plots have no axes since the data were projected in Euclidean distances to represent the sample dissimilarity. Therefore, such projection is a visual map of the relative distance between the points and the grids were just intended to facilitate data visualization. The low dispersion of the points of the samples and water in the plot of IDMAP indicates a satisfactory precision of the replicate measurements and the absence of nonspecific adsorptions

on the electrode surfaces. Thereby, the renewal of the insulated electrodes was dispensed with during the assays. The insulation of the capacitor metals (stainless steel) with dielectric films is of paramount importance for preventing such nonspecific adsorptions. Strikingly, a good correlation among the samples was achieved with an S of 0.95. Parallel coordinate plots showed that 84% of the frequencies assisted the classification as shown in the Supporting Information. Since the use only of these frequencies did not improve the classification, all of the frequency values were used in the multivariate data processing. Additionally, the single response e-tongue was reproducible in classifications for distinct days, whose S

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Figure 3. Classification of the SiO2NPs. Spectra of real capacitance (C) (A) and IDMAP plot employing the chip of capacitors with different dielectrics (B). IDMAP plot using the e-tongue of capacitors with different metals and SiO2 as dielectric (C). Spectra (D) and distinction (E) of the SiO2NPs in the multiplex tests using a pair of sample channels with different diameters, namely, 9. ± . µm (C1) and . ± . µm (C3). In (A,B), NPs and W mean nanoparticles and water, respectively. In (A-C), the functionalized SiO2NPs are identified by the molar ratios of TEOS:TMSPU, termed 0.5, 1.0, 1.5, and 2.0 (corresponding to the ratios of 1.0:0.5, 1.0:1.0, 1.0:1.5, and 1.0;2.0, respectively). In (a,d), the identification of each spectrum (average of three spectra showing very small confidence intervals) is displayed below the graphics. Insets in (A,D) show ratios of the capacitance spectra of the NPs in relation to water. In (D,E), while 1 means the pumping of only water in the chips, the other sets are identified according to the pumping of 0.5, 1.0, and 1.5 into C1 and C3, respectively. The sets with distinct samples are 2 (0.5;0.5), 3 (1.0;1.0), and 4 (1.5;1.5), whereas the pairs of clusters for the same two NPs but pumped into the channels with the samples swapped are 5 (1.0;0.5) and 6 (0.5;1.0), 7 (1.5;0.5) and 8 (0.5;1.5), and 9 (1.5;1.0) and 10 (1.0;1.5) as highlighted in (E) by dashed red lines.

values were maintained at 0.95. The absence of electrode passivation and the high uniformity of the oxide films on the electrode surfaces are crucial for this reproducibility that is mandatory for the proposed goal, i.e., the routine monitoring of nanomaterials as they avoid the need to make new multivariate calibrations of the method before each measurement. Another analysis that indicates reliability of the e-tongue data was the distinction of the SiO2NPs with the same microchip, but using the capacitors composed of different SiO2-insulated metals. Despite the use of different electrodes, the IDMAP profiles of the two e-tongues were similar to each other in relation to the relative distances among the samples as illustrated in Figure 3C. For instance, the closest points represented the NPs chemically modified with TMSPU 1.0 and 2.0 (see such definitions in the caption of Figure 3). Using the capacitors with different metals, S was as high as 0.96 and the data for water (cluster not shown) revealed low dispersion, once again suggesting a good precision of the replicates and ab-

sence of electrode contamination. In the next assays, only the results obtained by electrodes with distinct oxide dielectrics are described since the use of the different metals did not lead to improvements on the classifications (data not shown). As advantage, the microwires insulated by distinct dielectrics need only one step of thin film deposition, whereas the microwires coated with different metals need two of these steps. One should also emphasize the relative distances in the IDMAP plots were not consistent with the functionalization levels of the SiO2NPs obtained by TGA. This discrepancy can be attributed to the changes in polarization ability of the dispersions induced by functionalization, which change the capacitances attained by the e-tongue, but are not measured by TGA. The ability to make the distinction of SiO2NPs with only subtle differences in their dimensions and functionalization levels confirms the high sensitivity of the e-tongue with capacitors in parallel. The differences in the percentage of organic

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species on the SiO2NPs, for instance, were only 3.0% and 0.2%. Moreover, an e-tongue with the same electrodes forming an association of parallel resistors was not able to make this distinction presenting S of − . only (Supporting Information). The use of an e-tongue for simultaneous multivariate distinctions of different samples (multiplex) is addressed herein for the first time. The development of platforms with ability to perform several analyses simultaneously is still a challenge and increasingly valuable feature to decrease the analysis time. While electrochemical methods provide multiplex determinations utilizing diverse working electrodes,38 traditional etongues are not able to conduct such assays because of the subsequent measurements that are made for each one of the distinct sensing units. Our e-tongue of parallel capacitors provided multiplex classifications of SiO2NPs (1000.0 ppm) utilizing the same sensing units (different dielectrics on stainless steel) inserted in two devices. Sets of two samples were classified simultaneously among three dispersions of SiO2NP modified with the TMSPU ratios of 0.5, 1.0, and 1.5 (see the definition of these numbers in caption of Figure 3). The major challenge in this application is to categorize sets with the same two nanomaterials, but pumped into the channels with the samples swapped. For this purpose, the devices showed sample channels of different diameters to diversify the detection areas (sample/electrode contact) and, then to yield specific capacitance spectra as a function of the channel where the sample was inserted. Thereby, the multiplex platform was able to identify in which channel each sample was located. The resulting multiplex data confirmed the importance in response diversification for the quality of multivariate classifications. First, the samples were tested using devices with channels of 768.2 (C2) and . µm (C3) in diameter. S was 0.82, which means a clear classification. However, while the sets with distinct samples to each other were easily separated in the IDMAP plot, the pairs of sets with the same NPs but pumped in the chips with the samples swapped showed clusters with unclear distinction (Supporting Information). The use of channels with largest differences in diameter ( 9. µm, C1, and . µm, C3) promoted a best classification quality as revealed in the real capacitance spectra and IDMAP plot of Figure 3D,E. One should also highlight that each cluster in the IDMAP plot is not correlated to one sample, but to a pair of samples that were simultaneously assessed. S was improved to 0.87 with a strong distinction even of the pairs with the same samples, thus proving the potential of the method for effective multiplex multivariate monitoring. The insets in Figure 3A,D show the ratios of the equivalent capacitance spectra of the NPs in relation to water spectrum to highlight the differences between the samples. In these images, it is possible to see that the alterations in real capacitances happened at low and medium frequencies (1 to 104 Hz) as cited above. Characterization of the oxidized MWCNTs. Different samples were produced changing the time of oxidation in nitric acid, namely, 6, 12, 24, and 48 h. As reported in the literature,19 the industrial grade MWCNTs (raw sample) used herein consist of nanotubes with irregular agglomerates, but uniform individual lengths that commonly ranged from 3 to 6 µm according to FE-SEM images. Their diameters changed from 10 to 40 nm. Catalyst residues were observed in the EDS

spectrum through the presence of peaks assigned to oxygen, iron, and aluminum. The paucity of covalently bonded-oxygen on raw sample sidewalls was identified by energy-filtered TEM. Oxygenated groups are expected to be covalently bonded on the oxidized MWCNT surfaces as it was indeed observed in Figure 4, which illustrates the characterization data of the carbon nanotubes. In acid oxidation, the carbon-carbon covalent networks of the graphitic layers are broken leading to insertion of oxygen as lactone, carboxyl, and phenolic groups as shown in Figure 4A. These groups are usually exploited for further processes of dispersion in hydrophilic solvents16 and chemical functionalization.39,40 SEM images such as those in Figure 4B did not reveal differences among the nanotube samples. The oxidation of MWCNT produces oxygen groups that provide negative charge on nanotube surfaces as confirmed by zeta potential analyses (Supporting Information). The negative potentials of the oxidized carbon nanotubes were larger than − mV showing that the MWCNTs had a good colloidal stability in water. In fact, no sedimentation was observed even after 120 min according to photos and absorbance data of the MWCNTs in different times from their dispersion at ultrasound bath for 10 min (Supporting Information). The oxidation extents were tested by TGA and XPS as depicted in Figure 4C-F. The functional groups created by oxidation decompose at temperatures of up to ºC.41 Therefore, the mass losses by TGA in this temperature are relative to the number of oxygenated groups on surfaces of the carbon nanotubes. However, the percentages of such mass losses did not show a direct relationship with the oxidation time, indicating a poor sensitivity of TGA for this characterization. By contrast, the differences between the oxidized MWCNTs were successfully assessed by XPS that was used to calculate the atomic ratios of oxygen from O 1s peaks (roughly 530 eV). While the oxygen ratio of the raw sample was . % (± . ), the ratios of the oxidized MWCNTs increased with the oxidation time ranging from 6.5% to 11.0% with a gap lower than 2.5% in all of the situations. Classification of the oxidized MWCNTs. Analogously to what was verified for the SiO2NPs, the oxidized MWCNTs presented only subtle differences in their physicochemical surface properties. Only the sophisticated XPS technique provided reliable information on the MWCNT oxidation levels. The successful classification of these nanotubes by our etongue once again confirmed its high sensitivity. The changes in real capacitances of the MWCNTs in Figure 5A were observed mainly at low and medium frequencies as well. The raw sample was not analyzed since the dispersions prepared in water were not stable. In accordance with parallel coordinate plots, 84% of the frequencies assisted the classification as displayed in the Supporting Information. Taking up all the frequencies, the distinction of the fingerprints could be realized with success as noted in the IDMAP plot of Figure 5B for which S was 0.93, confirming the efficient distinction of the samples. The low dispersion of the triplicates of each sample and water (control sample to assess the electrode contamination) once again indicated good precision of the analyses and absence of nonspecific adsorptions on microwire surfaces. Thereby, similar to what happened with the SiO2NPs, the renewal of the insulated electrodes was dispensed with for the analyses to MWCNTs. The insets in Figure 5A,C show the

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ratios of the equivalent capacitance spectra of the CNTs to water spectrum. Similar to the NPs, the alterations in capacitance were observed especially between 1 and 104 Hz. The relative distances in the IDMAP plot were not consistent with the oxidation extents by XPS. Since the latter method ascertains the surface chemical composition, similar to what was noted for the nanoparticles, this discrepancy can be attributed to the alterations in dispersion polarization ability induced by oxidations that change the e-tongue data, but are not captured by XPS. Furthermore, the IDMAP profile recorded by the capaci-

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Figure 4. Characterization of the modified MWCNTs. Schematic diagram of the oxidation of the nanotubes in nitric acid (A), SEM images of the nanotubes oxidized for 6 and 48 h (B), TGA plots (C), percentage mass loss of the functional groups generated by MWCNT oxidation at ºC by TGA (D), XPS survey spectra (E), and percentage atomic ratios of oxygen by XPS taking up the O 1s peak (F). The images of the MWCNTs in (A) were reproduced with permission (reference 39 Copyright 2015, Elsevier). In (B), the black and white scale bars mean µm and nm, respectively. All the definitions of the MWCNTs (related to their oxidation times) in (C) are also valid for (E).

tors of distinct SiO2-insulated metals (Supporting Information) was once again similar to the behavior obtained with capacitors of different dielectrics (Figure 5B). In both these situations, the closest clusters were correlated to the nanotubes oxidized for 12 and 48 h, whereas the samples oxidized for 6 and 24 h were the dispersions with best discrimination. These data confirm the reliability of the e-tongue analytical signals. We further tested the effects of the sample concentration and dielectric film thickness on the e-tongue performance as shown in the Supporting Information. First, experiments for MWCNTs at lower concentration (10.0 ppm) revealed a decrease in capacitances, as expected according to the drop in electric field generated by the dielectric samples. In addition, the sensitivity was decreased and the classification ability was poor with S of 0.67, thus showing the damage in e-tongue performance with the reduction in CNT concentration. In this case, the IDMAP profile concerning the sequence of the oxidized samples in the library was distinct on that plot obtained for 500.0 ppm of nanotubes. Conversely, the data for the latter concentration with decreased thickness of the four oxide dielectric films to 400 nm exhibited a raise in the capacitance values as expected from Gauss’ law.29 The clusters in IDMAP plot had a similar behavior to those for 800 nm films and the sensitivity increased significantly between the samples oxidized for 6 and 24 h. However, while the classification of the

MWCNTs was satisfactory, S (0.86) was lower than the value acquired with 800 nm dielectric films (0.93). Multiplex classifications of the MWCNTs (500.0 ppm) were realized using C1 and C3 channels as well. Sets of two samples were classified among three dispersions with distinct oxidation times, namely, 6, 12, and 24 h. The equivalent real capacitance spectra and IDMAP plot in Figure 5C,D confirmed a clear multivariate distinction with S calculated as 0.82. Both the sets with different samples and same dispersions but inserted in the chips with the samples swapped presented clusters with a satisfactory classification.

CONCLUSIONS

The use of a microfluidic e-tongue for classifying nanomaterials with very similar surface properties has been demonstrated taking functionalized SiO2NPs and oxidized MWCNTs for proof- of-principle assays. Harnessing the data of capacitance from e-tongue treated with the multidimensional projection technique IDMAP, the SiO2NPs and MWCNTs samples could be distinguished in spite of the very small surface modifications induced by the distinct extents of functionalization with TMSPU (for the SiO2NPs) and oxidation time (for the MWCNTs). For instance, FTIR and TGA did not provide accurate information about the surface alterations of the

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SiO2NPs and MWCNTs, respectively. Other advantages of the single response microfluidic e-tongue made with an association of capacitors in parallel are discussed next. i) An indefinite number of sensing units can be used, thus

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Figure 5. Classification of the modified MWCNTs. Spectra of real capacitance (C) attained by the device of capacitors with different dielectrics (A) and IDMAP plot for classification of individual samples (B). Spectra (C) and distinction (D) of the MWCNTs in multiplex assays using C1 and C3 as sample channels. In (A,B), W means water and the nanotubes are identified according to their oxidation time, namely, 6, 12, 24, and 48 h. In (A,C), the identification of each spectrum (average of three spectra with very low confidence intervals) is shown below the graphics. Insets in (A,C) depict the ratios of the capacitance spectra of the MWCNTs in relation to the water. In (C,D), while 1 represents the pumping of only water in the microchips, the other sample sets are identified in agreement with the pumping of 6, 12, and 24 (mean the oxidation time in hours) in C1 and C3, respectively. The sets with different samples are 2 (6;6), 3 (12;12), and 4 (24;24), whereas the cluster pairs with the same two nanotubes but pumped into the channels with the samples swapped are 5 (6;12) and 6 (12;6), 7 (6;24) and 8 (24;6), and 9 (12;24) and 10 (24;12) as highlighted in (D) by dashed red lines.

leading to the distinction of even complex samples from a single signal. ii) Remarkable improvements in simplicity, reproducibility, and fastness were obtained in relation to the conventional e-tongues. iii) There is no need of active functional units to get, e.g., contrast in microscopy or faradaic current in electrochemical techniques. The use of impedance also eliminates the need to dilute the samples in electrolytes. Instead, the nanomaterials were simply prepared in water that is usually the ideal dispersion phase. The electric readouts were acquired in flow in a label-free mode. iv) The contactless electrodes (inedited assembly for e-tongues) and uniform oxide coatings led to robust and reproducible tests. v) The electrodes can be readily renewed in situations of surface passivation, fouling, or contamination by simply sliding the microwires along the channel for sample.21 vi). The e-tongue possess ability to conduct the scrutiny of functionalized nanomaterials in real time. vii) Also demonstrated for the first time, the single response e-tongue afforded multiplex monitoring. Since the capacitances were recorded at the same frequency range, simultaneous measurements of distinct samples were possible from the same electrode set, which is advantageous compared with electrochemical methods where distinct working electrodes are necessary due to the peak potentials specific of each analyte. Simultaneous tests were performed herein for

two samples, but it can be extended to more samples using additional chips with distinct channels to each other. The challenge to analyze simultaneously three or more samples is related to the extensive number of assays for constructing the IDMAP library. Considering four samples, e.g., 768 measurements (256 analyses in triplicate) would be required. Regarding the fabrication of the microfluidic chip, we used a bondless and cleanroom-free method, which represent relevant advantages compared with conventional prototyping techniques toward glass (photolithography, etching, and thermal bonding) and polymeric (photolithography, soft lithography, and bonding by plasma oxidation, surface modification, or sandwich mode) devices.42 Similar to these techniques, PSR approach presents poor productivity (it lasts approximately roughly 3 h to produce one e-tongue device) and massproduction compatibility. In this case, one potential alternative is the use of paper-based chips with both the channels and electrodes fabricated by direct printing using wax printer and pencil, respectively.43,44 The main purpose of utilizing an e-tongue for routine monitoring of nanomaterials is obviously not to replace the surface analysis techniques (e.g., SEM and XPS) that are commonly required to determine surface properties with precision. Instead, the goal is to provide a quick screening and quality-

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monitoring procedure to assess nanomaterials in mass production scenarios in industry or day-to-day analyses in the lab. The classification of some complex samples requires the functionalization of the electrodes with receptors for specific interactions.8 This modification could be made in our e-tongue on the dielectric surface. In this case, we believe that the use of different metals insulated by the same dielectric film would be the best choice. One should also mention that the ability of classification of the e-tongue for such real applications, including prediction capacity, could be further raised by inserting more samples in the IDMAP library, which we believe would have a large impact in the commercial use of nanomaterials.

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ASSOCIATED CONTENT

Supporting Information Available: The following files are available free of charge. Characterization of the microwires as well as characterization and classification of the nanoparticles and nanotubes.

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AUTHOR INFORMATION

Corresponding Author

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* E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

Financial support for this project was provided by Petrobras (Grant 2015/00301-6) and Fundação de Amparo à Pesquisa do Estado de São Paulo (Grants 2012/15543-7, 2013/22429-9, 2013/14262-7, 2014/24126-6, and 2015/25406-5). Rafael Defavari from Centro Nacional de Pesquisa em Energia e Materiais (CNPEM) is thanked for taking the photos. Lastly, the authors also thank CNPEM for the use of its facilities.

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Monitoring the Surface Chemistry of Functionalized Nanomaterials

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