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Article Cite This: Langmuir 2019, 35, 8823−8828
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Cryoporometry in Femtoliter Volumes by Confocal Raman Spectroscopy Katarzyna Piela, Eric C. Tyrode, and Istvań Furo*́ Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden
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S Supporting Information *
ABSTRACT: The properties of porous material are largely dependent on the size, shape, and connectivity of the pores. Here, we present a method based on confocal Raman spectroscopy to quantify porosity using a cryoporometric approach. We show that the phase transition of water imbibed in porous silica can be accurately determined using two different, but complementary methodologies. The first one relies on integrating the temperature-dependent spectral intensities across the whole OH (H2O) or OD (D2O) stretching region. The second, more quantitative approach, deconvolutes the spectral contributions within the pores in terms of liquid and solid fractions. The results show the expected reciprocal dependence of the average phase transition point with pore size, as well as the typical hysteresis between the freezing and melting transitions. One of the key advantages of the confocal Raman approach is its high spatial resolution, with sampling volumes starting from just a few femtoliters, opening the possibility of mapping the structure in heterogeneous porous materials.
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INTRODUCTION Nature and human technology are abundant with porous materials whose behavior and function depend intimately on the size, shape, and connectivity of the constituting pores. However, characterizing porous materials is neither easy nor straightforward.1−3 Although there is a large number of techniques available, they suffer from significant shortcomings that ultimately lead to the preference of a given method depending on the particular problem and/or material under investigation. The direct implication is that different methods often yield results, such as pore size distributions, that are not completely consistent with each other.4,5 Additionally, and to some extent interconnected to the previous point, there is a need for alternative methodological approaches that may fill niches not well served by currently available techniques. Our findings here regard this second point. Broadly speaking, structural methods for characterizing pore structures may be divided into two classes. The first one includes direct imaging methods.6−10 If performed in 3D mode such as in X-ray micro-CT experiments,6−8,11 a complete characterization of the porous structure can, in principle, be obtained. Disadvantages are limitations regarding sample size, pore size range, as well as the type of accessible materials, where the latter point depends on the contrast difference between the pore walls and, if any, the pore-filling material. Instrumental access is also limited for synchrotron-based experiments.12 Hence, current direct imaging methods leave the scene open for more traditional porometric (also called as porosimetric) methods that are more widely available, have a higher throughput, and can be less dependent on the physical characteristics of the porous materials. These porometric © 2019 American Chemical Society
methods constitute the second class. Typically, these methods have a common elementthey detect some physical phenomenon that, in pores of a particular structure, take place under conditions different from that in bulk.1−3 One example is gas sorption porosimetry3,13 where the porestructure-dependent shift of gas−liquid equilibrium (and, on a connected manner, the gas adsorption on pore walls) is explored. Our specific current proposal belongs to methods collected under the umbrella of cryoporometry (sometimes also called thermoporometry), which rely on detecting shifts in the temperatures of solid−liquid phase transitions, freezing and melting,14,15 that are dependent on the size and shape of the pores that confine the material exhibiting the observed phase transition. The type of suitable pore-filling materials is manifold, with water being an obvious option. One particular advantage with cryoporometric methods is that they work for such porous systems (e.g., hydrogels, though with known limitations4,16) that attain their native structure in their liquidfilled state. Regarding the method for detecting freezing and melting of the pore-confined materials, the two main detection tools are either calorimetric methods,17−19 such as differential scanning calorimetry, or NMR spectroscopy,16,20 though there are several other, less frequent approaches, like X-ray diffraction.21 What we present here is a different detection method based on confocal Raman spectroscopy, henceforth and in this context referred to as “confocal Raman cryoporometry”. First, we detail Received: March 12, 2019 Revised: April 29, 2019 Published: June 12, 2019 8823
DOI: 10.1021/acs.langmuir.9b00735 Langmuir 2019, 35, 8823−8828
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Ireland). In spite of using a corrected microscope objective with a relatively low NA and a pinhole with a diameter of less than 0.3 airy units, our confocal system suffers, though to a limited extent, from the same depth profiling limitations as other confocal Raman microscopes.25 Hence, the confocal volume probed depends on the axial position of the examined particle relative to the window/ice interface, but is typically in the range of a few tens of femtoliters. Specifically, the probed volume increases with the increasing distance of the focal point from the window, varying from ∼2 fL close (within 50 μm with particle sizes as here, this may correspond to particles in direct contact with the window) to the window surface (resolution of ∼0.4 μm in the lateral direction and ∼3 μm in the axial direction),26 up to a few hundred femtoliters 1 mm away from the surface (see the Supporting Information for detailed calculations). To facilitate the selection of the individual particles to be investigated, the temperature in the cell chamber was set above the respective average pore melting temperatures (if filled by D2O, approx. 274 K for CPG 15.6 nm, and approx. 276 K for CPG 50.7 nm, respectively), but below the bulk melting point (277.0 K for D2O). This ensures that the pore-filling water is liquid inside the particles, yet solid outside. Only single and isolated particles located between 100 μm and up to 800 μm away from the silica/ice interface were chosen for the measurements (see Figure 1). Their location was
the actual methodology used to determine the freezing and melting inside pores as a function of temperature and estimate the actual proportion of the liquid and solid phases along the phase transition. Finally, we elaborate on what novel niches our proposed method is uniquely suitable for.
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EXPERIMENTAL SECTION
Materials and Sample Preparation. The porous materials we used to demonstrate the method were controlled pore glasses CPG (CPG Inc.) with nominal pore diameters of 15.6 and 50.7 nm; see material properties compiled elsewhere.22 The matrix consists of silica into which a random interconnected tubular network of pores is etched. The CPG particles were irregularly shaped, with typical average diameters in the range of 100−150 μm. Before use, the particles were sonicated for 1 min in absolute ethanol (VWR BDH Chemicals), rinsed several times with ultrapure water obtained from an Integral 15 Millipore filtration unit (resistivity of 18.2 MΩ·cm, total organic carbon
