Article pubs.acs.org/ac
Topographical and Chemical Imaging of a Phase Separated Polymer Using a Combined Atomic Force Microscopy/Infrared Spectroscopy/ Mass Spectrometry Platform Tamin Tai,† Orsolya Karácsony,† Vera Bocharova,‡ Gary J. Van Berkel,† and Vilmos Kertesz*,† †
Mass Spectrometry and Laser Spectroscopy Group, Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6131, United States ‡ Soft Materials Group, Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6131, United States S Supporting Information *
ABSTRACT: In this paper, the use of a hybrid atomic force microscopy/infrared spectroscopy/mass spectrometry imaging platform was demonstrated for the acquisition and correlation of nanoscale sample surface topography and chemical images based on infrared spectroscopy and mass spectrometry. The infrared chemical imaging component of the system utilized photothermal expansion of the sample at the tip of the atomic force microscopy probe recorded at infrared wave numbers specific to the different surface constituents. The mass spectrometrybased chemical imaging component of the system utilized nanothermal analysis probes for thermolytic surface sampling followed by atmospheric pressure chemical ionization of the gas phase species produced with subsequent mass analysis. The basic instrumental setup, operation, and image correlation procedures are discussed, and the multimodal imaging capability and utility are demonstrated using a phase separated poly(2-vinylpyridine)/poly(methyl methacrylate) polymer thin film. The topography and both the infrared and mass spectral chemical images showed that the valley regions of the thin film surface were comprised primarily of poly(2-vinylpyridine) and hill or plateau regions were primarily poly(methyl methacrylate). The spatial resolution of the mass spectral chemical images was estimated to be 1.6 μm based on the ability to distinguish surface features in those images that were also observed in the topography and infrared images of the same surface.
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particular combination of surface sampling process, ionization method, and mass analyzer used will determine the surface types and chemical species that can be analyzed and the specific limits of detection and degree of chemical detection specificity. MSI, albeit destructive, typically provides greater chemically specific information about a sample than the spectroscopic methods but, at a minimum, can be a complementary approach to spectroscopy and AFM techniques. When combining MS and AFM, the AFM cantilever probe, in addition to performing the typical AFM physical measurements of a surface, can be exploited to sample from a surface by various mechanisms (e.g., field evaporation, thermal desorption or thermolysis, and near-field laser ablation) and in some cases ionize the sampled material for mass spectral detection.11 Because the sampling phenomena takes place at a spatial scale the size of the cantilever probe tip, low micrometer to nanoscale sampling is possible. By utilizing the AFM cantilever probe in this manner, correlated topography, physical, and chemical images of a surface can be produced in a relatively straightforward manner for the sample surface of interest.
nalytical tools that can provide spatially resolved sample surface morphology, or other physical information, as well as chemical information at the very low micrometer to nanometer scale, are inherently useful. For example, such correlated, or “multimodal”, information is beneficial for characterizing and understanding the composition and function of chemical interfaces in diverse fields like chemical science, material science, biology, and geology.1−4 One important tool used in this correlated, multimodal imaging pursuit has been atomic force microscopy (AFM).5,6 Topography and a wide range of physical properties of a surface, such as electric, magnetic, and thermal properties, can be obtained and correlated with a single AFM instrument and sample. However, chemical characterization is not possible with AFM alone.7 To overcome this limitation and extend the capability of the AFM technique, a variety of optical spectroscopy techniques, including infrared (IR)8 and Raman9 spectroscopies, have been used with AFM to create hybrid tools that can obtain spatially resolved chemical signatures from a sample surface.7,10 An alternative chemical imaging approach that has been combined with AFM is mass spectrometry imaging (MSI).11 MSI provides spatially resolved selective detection and identification within complex matrices, either through (accurate) mass measurements or through the use of tandem mass spectrometry (MS/MS).12,13 When performing MSI, the © 2016 American Chemical Society
Received: December 5, 2015 Accepted: February 8, 2016 Published: February 18, 2016 2864
DOI: 10.1021/acs.analchem.5b04619 Anal. Chem. 2016, 88, 2864−2870
Article
Analytical Chemistry
upon heating. Both chemical detection methods were found to be internally consistent with each other, showing that either chemical imaging approach could be used independently to map the polymer distributions in this relatively simple twophase thin film. The basic AFM/IR/MS instrumental setup is discussed as well as the procedure and software developed to precisely correlate the topography, IR, and mass spectral images. Comparison of features observed in these images showed the mass spectral spatial imaging resolution to be about 1.6 μm.
An AFM cantilever probe controlled surface sampling process that we have been advancing for hybrid AFM/MS imaging is atmospheric pressure (AP) thermal desorption/ thermolysis.11 The basic proximal probe thermal desorption/ thermolysis approach using AFM was pioneered over a decade ago by Price and co-workers using AP sampling and vacuumbased ionization with MS, but MSI was not realized at that time.14−18 Recently, we have advanced the surface sampling/ ionization and analysis approach to enable MSI, specifically by using nanothermal analysis (nano-TA) AFM probes.19−23 These probes are heated to the appropriate temperature and placed in close proximity to, or in actual contact with, the surface to locally desorb intact molecular species or liberate into the gas phase products of the thermolysis of the surface material. Obviously, samples that can not be liberated into the gas phase with heat are not amenable for this kind of surface sampling. The gas phase species liberated from the surface under ambient conditions are then ionized by electrospray ionization (ESI)24 or atmospheric pressure chemical ionization (APCI)25,26 and analyzed using MS. With these hybrid instruments, we have been able to achieve correlated AFM topographical images of surfaces with mass spectral signal from 250 nm spot samples24 and a mass spectral imaging resolution of 1.5 to 2.6 μm,26 as well as correlated topographical, band excitation nanomechanical, and mass spectral chemical images.26 In this paper, we demonstrate the capability to obtain and precisely correlate AFM topography and chemical images of a thin film polymer surface based on IR spectroscopy and MS using a single hybrid AFM/IR/MS platform. This capability is demonstrated using a phase-separated polymer blend comprised of poly(2-vinylpyridine)/poly(methyl methacrylate) (P2VP/PMMA) thin film as a model system (Scheme 1).
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EXPERIMENTAL SECTION Samples and Solvents. P2VP (Mw = 9100, Mw/Mn = 1.05, Tg < 368 K) was purchased from Scientific Polymer Products, Inc. (Ontario, NY, US). PMMA (Mw = 126 000, Mw/Mn = 1.15, Tg= 378 K) was purchased from Polymer Source, Inc. (Dorval, QC, Canada). Anhydrous chloroform (≥99%) was purchased from Sigma-Aldrich (St. Louis, MO, USA) and used without further purification. Polymer Thin Film Preparation. A polymer solution was created by adding 300 mg of PMMA and 300 mg of P2VP into 6 g of chloroform in a scintillation vial to obtain an ∼4.5 wt % of each polymer in the solution. A 10 mm × 10 mm silicon wafer was cleaned by ultrasonication in acetone and then in methanol for 5 min and dried in a stream of nitrogen. A 10 μL aliquot of the solution was dropped on the clean silicon wafer which was then spun at 3500 rpm for 60 s using an EDC-65015 spin coater (Laurell Technologies, North Wales, PA, USA). This procedure resulted in an ∼2 μm thick polymer film, as measured by a Tencor P-10 profilometer (Tencor, Miliptas, CA, USA). The film was annealed in a vacuum oven at 380 K (107 °C) for 1 h. The annealing process above the glass transition temperature helped the two blended polymer phases to separate. AFM/IR/MS System and Imaging Procedure. A LTQ XL mass spectrometer (Thermo Fisher Scientific., Waltham, MA) and a nanoIR2 AFM instrument (Anasys Instruments, Santa Barbara, CA, USA) were used in this work. Material liberated from a polymer surface via thermolysis induced by the heated AFM nano-TA probes was ionized and transferred to the mass spectrometer via an in-line vapor extractor/corona discharge APCI source that has been described in detail elsewhere.26 Compared to that setup, a slightly longer (8 cm) extractor capillary was used and micrometer adjusters were added to enable fine position control of the capillary relative to the AFM probe. Schematic diagram and photos of the setup are shown in Scheme S1. IR imaging experiments utilized a tunable optical parametric oscillator (1 kHz, 10 ns pulse width) and gold coated nanoIR2 cantilever probes with a spring constant in the range of 0.07− 0.4 N/m (model PR-EX-NIR2, Anasys Instruments). Nanoscale IR spectroscopy was achieved by detecting photothermal expansion of the sample at the tip of the nanoIR2 AFM probe as a function of IR wavenumber.29 Absorption of IR light by the sample caused a rapid thermal expansion resulting in resonant oscillations of the AFM cantilever. Generation of an IR image required a preparatory topography image of the phaseseparated polymer to be acquired to locate spots specific for each phase. IR spectra were collected at these particular spots followed by identification of peaks in the IR spectra unique to each polymer (see Figure S1). In an IR imaging experiment, the sample was scanned and continuously exposed to a beam of IR light at one of these wave numbers specific for the substance to
Scheme 1. Chemical Structures of the Two Polymers, Their IR Detection Wave Numbers, and the Species Detected in the Gas Phase by MS Following Nano-TA Thermolysis and Ionization by APCIa
MS/MS (m/z 101 → 73) was used to improve the detection specificity for methyl methacrylate.
a
The chemical images based on photothermal IR spectroscopy differentiated the polymers on the basis of absorbance peaks characteristic of functional groups unique to each polymer (C− N stretching vibration of pyridine ring in P2VP27 and C−O−C single bond stretching vibration in PMMA28). The mass spectral-based chemical images were derived from the detection of the distinct polymer monomers (methyl methacrylate and 2vinylpyridine for PMMA and P2VP, respectively) that were liberated into the gas phase by thermolysis of the polymers 2865
DOI: 10.1021/acs.analchem.5b04619 Anal. Chem. 2016, 88, 2864−2870
Article
Analytical Chemistry
Figure 1. Schematic illustration showing the instrumental components of the AFM-based (a, top) IR and (b, top) MS imaging experiments. (a, bottom) 2D IR chemical images for PMMA and P2VP and the corresponding topography image and (b, bottom) the topography image and corresponding 2D mass spectral chemical images for PMMA and P2VP. 3D overlays of (c) IR and (d) mass spectral chemical image information for PMMA and P2VP on topography. Spatial location color in the 3D overlays was obtained by mixing colors representing independently normalized chemical signal of PMMA and P2VP at that location on black-to-blue and black-to-yellow linear color intensity scales, respectively. Vertical and horizontal axes of the color scale squares represent these independently normalized chemical signals of PMMA and P2VP, respectively, with the square showing the mixed colors.
USA). This external software then modified the nano-TA probe heating voltage in the AFM software on each line scan trace (10 V, thermolysis at >370 °C) and retrace (2 V, about 70 °C to minimize heat-induced mechanical deformation of the cantilever). Mass spectra for imaging were recorded in an offresonance collision induced dissociation (CID) mode (normalized collision energy (CE) of 20% with mass-to-charge ratio (m/z) 103.5 selected as the precursor ion and m/z selection window of 7) with a 50 ms injection time. These conditions led to the trapping of the protonated monomers of both P2VP (m/ z 106) and PMMA (m/z 101) but only the dissociation of the protonated PMMA monomer ion (m/z 101 → 73). See Figure S2 for representative mass spectra of areas corresponding mainly to P2VP and PMMA. This detection scheme provided the fastest data acquisition speed possible on this ion trap instrumentation (150 ms/scan) while still being able to accomplish detection specificity of the PMMA through the use of MS/MS. Lane scan speed and lane spacing were selected
be imaged. The result was an IR absorbance image where the signal intensity at each spatial location (IR data pixel) corresponded to the IR absorbance at that location. It must be noted that evaluation of IR resolution (sub-100 nm in recently published routine applications by AFM/IR30−32 and by AFM/scattering scanning near-field optical microscopy (SNOM)33) and optimization of IR signal-to-noise ratio of the nanospectroscopical imaging modality were not the purpose of this work. AFM/MS imaging employed nano-TA AFM probes (model PR-EX-AN2-300, Anasys Instruments). The position of the nano-TA AFM probe tip relative to the extraction capillary was visualized for accurate positioning using a camera connected to a video monitor. During mass spectral imaging experiments, the digital output of the Anasys controller signaled trace/retrace directional changes to an in-house-developed software (AFMAssistant) via a digital input port of a USB-1208LS data acquisition device (Measurement Computing, Norton, MA, 2866
DOI: 10.1021/acs.analchem.5b04619 Anal. Chem. 2016, 88, 2864−2870
Article
Analytical Chemistry
polymer and release the polymer specific monomers into the gas phase (Scheme 1). These gas phase monomers were drawn by the vacuum pull of the mass spectrometer into an extractor capillary placed within ∼1 mm of the AFM probe and into an in-capillary corona discharge APCI source and then the mass spectrometer. Ambient air (N2, O2, and H2O vapor) was drawn into the source along with the volatilized surface materials so that in positive ion mode the major reagent ions were normally protonated water/water clusters.39 As such, the major ion− molecule reaction with the vapor material sampled was expected to be proton transfer resulting in the formation of protonated monomer molecules (M + H)+. Protonated 2vinylpyridine was detected at m/z 106. In the case of PMMA, MS/MS of protonated methyl methacrylate was utilized to add detection specificity, because of background chemical noise in this region of the mass spectrum (m/z 101 → 73; see Scheme 1). These two species were measured together in an offresonance CID experiment that resulted in the dissociation of the ion at m/z 101 to m/z 73 but left m/z 106 intact. This detection approach maximized the data acquisition speed at 150 ms/data point using the ion trap instrument. The intensity of each detected species at a specific location (mass spectral data pixel) was used to render the mass spectral chemical images. Because changing the cantilever probes in the AFM was required to acquire both IR and mass spectral chemical images from the same sample, it was necessary to follow a well-defined data acquisition procedure and to develop software to enable the precise automated spatial coregistration of the topography, IR, and mass spectral information. The data acquisition procedure (shown schematically in Figure 1) started by collecting IR images (simultaneously with topography images) of an area slightly larger (about 3−5 μm wider and taller) than the area of interest. The gold coated nanoIR2 probe was then replaced with the nano-TA probe. After cantilever probe replacement, the tip of the nano-TA probe was found to be positioned at very nearly the same position where the tip of the nanoIR2 probe was located before the probe change. We found the tip could be moved within about 10 μm of that location during the initial tip alignment procedure aided by simple visual observation of the tip through the optical microscope on the current AFM system. To minimize further any misalignment, a portion of a full topography image (∼10−30%) was acquired and the area scanned in the AFM software was adjusted on the basis of visual comparison of features in these partial topography images and the topography image acquired earlier using the nanoIR2 probe. Typically, only 3−4 iterations were needed to achieve a misalignment of
