Role of Glucose in Enhancing Stability of Aqueous Silica Gels Against

Article pubs.acs.org/JPCC

Role of Glucose in Enhancing Stability of Aqueous Silica Gels Against Dehydration Gérald Lelong,†,‡,* Rodolphe Heyd,† Georgia Charalambopoulou,§ Theodore Steriotis,§ Astrid Brandt,∥ Kevin Beck,† Marylène Vayer,† David L. Price,⊥ John W. Brady,∇ and Marie-Louise Saboungi†,* †

Centre de Recherche sur la Matière Divisée, CNRS, Orléans, France Institut de Minéralogie et Physique des Milieux Condensés, Université Pierre et Marie Curie, CNRS-UMR 7590, Paris, France § National Centre for Scientific Research “Demokritos”, Athens, Greece ∥ Helmholtz-Zentrum Berlin für Materialien und Energie, Lise-Meitner Campus, Berlin, Germany ⊥ Conditions Extrêmes et Matériaux: Haute Température et Irradiation, CNRS, Orléans, France ∇ Department of Food Sciences, Cornell University, Ithaca, New York 14853, United States ‡

ABSTRACT: The microstructure evolution of confined glucose solutions in silica gels can provide insights into the effect of sugars in protecting living organisms under extreme dehydration conditions. Aqueous silica gels with relatively small pore sizes capable of deforming under changes in environmental conditions are used here as a model system. In situ monitoring of the dehydration process − with and without the presence of sugar molecules − by optical photography, gravimetric measurements, small-angle neutron scattering, and atomic force microscopy reveals that sugar plays a crucial role in the mechanics and protection of the gel. In the absence of sugar, dehydration leads to considerable degradation, whereas the incorporation of large doses of glucose maintains the stability and robustness of the structure. A model is proposed to explain the time dependence of the dehydration process.

exceeding 20 nm,18,19 a result confirmed by recent in cellulo measurements.20,21 The present work aims to explain the protective effect played by sugars in a dehydration process. We have followed the evolution of the microstructure of aqueous silica gels incorporating a simple monosaccharide − glucose − as a function of sugar concentration and hydration level. The porous host was subjected to a dehydration process with and without the presence of sugar molecules, and its response was monitored in situ by optical photography, gravimetric measurements, small-angle neutron scattering (SANS), and atomic force microscopy (AFM). The results were analyzed by a simple model that provides a satisfactory explanation of the water diffusion during this complex process.

1. INTRODUCTION Certain living organisms are able to survive hostile environments in which life seems to be completely impossible. The adjustment of these living species to extreme cold or drought conditions has been associated with the secretion of carbohydrates.1−3 Several studies have focused on the role played by some mono- and disaccharides, such as glucose, sucrose, and trehalose, as well as their effect on the stabilization of biological structure and function.4−6 The alleged protective mechanism is still unclear, however, with several theories invoking direct or indirect interaction of the sugar molecules with nanostructural cellular elements (e.g., proteins and membrane lipid bilayers) that could allow them to act as a substitute for water.3,7−11 The actual response of sugar molecules is expected to depend on the geometry and size of the space available. In view of the so-called crowding ef fect in cells,12,13 the investigation of sugar behavior under confinement is a key issue for elucidating the bioprotective mechanisms. In this context, there have been some studies of silica/sugar composites,14−17 but only a few have addressed the effects of the confinement of sugar solutions in inorganic porous matrices. Host matrices with relatively small pore sizes are capable of deforming in response to changes in environmental conditions. Quasielastic neutron scattering results have demonstrated that molecular motions are not significantly affected by confinement for pore sizes © 2012 American Chemical Society

2. MATERIALS AND METHODS A series of aqueous silica gels containing either normal or partially deuterated D-glucose were prepared through a sol−gel process22,23 using as precursors tetraethyl orthosilicate (TEOS), water (either H2O or D2O), HNO3, urea, and glucose at a molar ratio of 7.2: 1110: 1.6: 0.025: x, where x = 0, 12.4, 19.6, 27.8 and 47.6 corresponding to gels with glucose concentration Received: August 30, 2011 Revised: April 5, 2012 Published: April 6, 2012 9481

dx.doi.org/10.1021/jp208377t | J. Phys. Chem. C 2012, 116, 9481−9486

The Journal of Physical Chemistry C

Article

Figure 1. Lateral view of gels with different initial glucose concentrations at different evacuation times.

concentration dependent since it is proportional to the number of hydrogen atoms. The surface morphology of both the dehydrated 0 wt % and 15 wt % gels obtained at the end of the gravimetric study was probed by AFM (Molecular Imaging Pico+).

in fully protonated solutions of 0, 10, 15, 20, and 30 wt %, respectively. TEOS (from Merck) was mixed with water and a small amount of HNO3 to reach pH ∼2. Following the completion of the alkoxide hydrolysis, urea, and a specific amount of D-glucose were added to the solution under vigorous stirring. The thermohydrolysis of the urea generates ammonia increasing the pH of the sol and polycondensing silicic acid into silica gel. To ensure complete gelation, the sols were left at 80 °C for 1 to 2 days. The partially deuterated glucose (C6H7D5O6) used for the SANS measurements was obtained by mixing normal D-glucose with an excess of D2O to replace the exchangeable hydrogen atoms by deuterium and then drying. The dehydration process was monitored by systematically recording the weight loss of gels with different glucose concentration upon the removal of water. A batch of the five starting gel compositions was stored in a desiccator connected to a vacuum pump. After applying a vacuum the specimens were weighed at 1 h intervals during the first 5 days, and then every 24 h for the remaining 95 days. SANS measurements were performed on all five samples of SiO2-D2O-glucose gels before and after dehydration at 20 °C, on the V4 instrument at BENSC (Helmholtz Zentrum Berlin für Materialien and Energie, Berlin, Germany). A neutron wavelength of 0.605 nm and three sample−detector distances (1, 4, and 16 m) were used, covering a Q range from 3.5 × 10−3 to 0.33 Å−1. Following the measurement of the as-produced gels, each of them was subjected to a sequence of dehydration steps under controlled temperature/humidity conditions to attain different levels of water content until the formation of practically dry films. A special in-house-built system operating isothermally, including a vacuum tight rig, a vacuum pump, a temperature controlled water bath, and appropriately modified leak-tight quartz cuvettes bearing glass valves, was used for this purpose. The measured 2D patterns were calibrated with water, corrected for background and empty-cell scattering, and finally radially averaged using the data reduction software BerSANS.24 After subtraction of the incoherent background, the intensity I(Q) was corrected in terms of the contrast factor, which is

3. EXPERIMENTAL RESULTS The macroscopic evolution of aqueous silica gels containing 0, 10, 15, 20, and 30 wt % of glucose as a function of pumping time is shown in Figure 1. Initially, the five gels were soft and fragile, and their appearance (Figure 1, first row) illustrates that the addition of sugar increases their transparency. After 720 h of dehydration, the gel without sugar is damaged irreversibly, whereas the gels containing sugar appear more resistive, with a stability that increases with glucose concentration. This trend is sustained throughout the period of the measurement. After 2200 h, the dried sugar-free gel is broken into many small chunks. Shrinking and cracking are also observed in the gel with 10 wt % sugar but to a much lesser extent, and the degradation is noticeably reduced when the glucose concentration reaches 30 wt %. Whereas the gel with no or very little sugar loses up to 85% of its volume on drying, the gel with 30 wt % glucose remains relatively intact maintaining not only its volume and clarity but also its texture and elasticity. Water acts as a plasticizer in the gels containing sugar by linking the carbohydrates together by multiple hydrogen bonds.25 Even at a relatively high level of dehydration, this gel retains almost its full volume due to steric effects caused by the presence of sugar molecules and by the entrapped water molecules acting as pillars between the carbohydrate chains reinforcing the silica backbone. The water loss was monitored quantitatively by systematic weighing during the dehydration process. The water content X(t) of the sample can be defined by: X (t ) =

m (t ) − m (∞ ) m (∞ )

(1)

where m(t) is the mass of the sample at time t and m(∞) that at the end of drying. The moisture ratio M(t) is then defined as: 9482



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Figure 2. Evolution of the moisture ratio M(t) (a) at short times up to 12 h, (b) at intermediate times up to 200 h and (c) at long times up to 2200 h, for different initial glucose concentrations. The labels TR, Q-CR, FR1, and FR2 represent the periods with different drying behavior described in Section 4.

M (t ) =

X (t ) X(0)

The very low loss rate (1.4 × 10−4%/h) for the gel with 30 wt % glucose is sustained for a prolonged period of ∼300 h representing the time needed to form nanochannels, through which the water is extracted, as previously suggested for trehalose and maltodextrin glassy systems.25−27 Part (c) of Figure 2 shows that the overall time needed for complete depletion of water increases significantly with the initial glucose loading. Whereas the sugar-free gel reaches the dry state after 330 h, the gel with 30 wt % sugar retains over 10% of the initial water content for a period of 1300 h, almost four times longer. SANS spectra taken before and after dehydration for the gels containing 15 and 30 wt % sugar are shown in Figure 3. The gel without sugar could not be measured after dehydration because the solution cracked (Figure 1); however before complete dehydration its spectrum was similar to those of the gels with sugar implying the presence of similar porous networks in the fully hydrated version. The SANS spectra (Figure 3) exhibit two regimes: (1) a regime with Q‑2 behavior indicating a fractal structure with a bulk fractal dimension Dbulk ≈ 2 typical of reaction-limited cluster−cluster aggregation28 and a mean particle radius ≈ 1.5 nm and (2) a regime with a Q−4 behavior indicating a surface fractal dimension Dsurface ≈ 2 implying the absence of surface irregularities.29 The SANS spectra confirm

(2)

The evolution of M(t) during the dehydration undergoes several stages. Part (a) of Figure 2 depicts the moisture ratio of the five gels during the first 12 h of each measurement. The step change observed in all cases at a very early stage (∼0.3 h) is attributed to the removal of excess water residing mainly on the external surface of the gels. This effect diminishes steadily with increasing sugar concentration and finally vanishes at 30 wt %. After the bulk water is removed, the five gels exhibit a similar behavior described by a dehydration rate of about 7 × 10−3%/h until approximately 15 h. These relatively fast desorption kinetics coupled with the consistent response of all the samples point to a mobile water phase. As dehydration progresses further, the moisture ratio behavior depends strongly on the initial glucose concentration, as seen in part (b) of Figure 2. After 33 h, the sugar-free gel continues to dehydrate but at a reduced rate (2.2 × 10−3%/h) indicating a transition from a mobile to a bound phase characterized by a change of slope in the desorption curve. The same transition is observed in the gels containing sugar, but the extraction rate after the transition decreases with increasing sugar content. 9483



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Figure 3. SANS spectra of gels with 15 wt % sugar (left) and 30 wt % sugar (right), before and after dehydration.

% glucose exhibits a smoother and more homogeneous surface composed of small white blobs uniformly dispersed on the surface. The corresponding phase image (lower figure) demonstrates clearly the dispersion of sugar aggregates embedded in the silica matrix. A homogeneous aggregation of the glucose starts to occur during the dehydration process but is limited by the presence of the siliceous backbone. Glucose seems to play the role of pillars in the maintenance and stabilization of the silica structure. A similar phenomenon has been observed in the nonsurfactant templated method used in the synthesis of some mesoporous materials.30,31

that the structural integrity of the gel is maintained when 15− 30 wt % of glucose is added. AFM results (Figure 4) on the dehydrated sugar-free gel and that with 15 wt % glucose confirm the low resistance of the

4. ANALYSIS AND DISCUSSION The fluid transport accompanying the vacuum drying of these gels is a complex process that involves multiple components: water evaporation, displacement of liquid and vapor due to the capillary pressure, diffusion of the liquid and the vapor, and so forth. These phenomena are obviously coupled and depend strongly on the temperature of the system and the evaporation surface which changes in a complex way during drying. Analysis of the drying curves of Figure 2 can elucidate the basic physical mechanisms governing the water transport and the role played by the glucose. Figure 5 shows the drying rate −dX/dt versus the moisture ratio M(t) for the gels with 0, 15, and 30 wt % glucose. The three curves have similar shapes showing that the nature of the physical processes involved does not depend on the sugar content, although the relevant physical parameters (dynamic viscosity, porosity, latent heat of vaporization, etc.) can vary considerably. As is often the case during the drying of porous media such as fruit and wood,32 two principal types of behavior can be distinguished: a constant-rate (CR) period (dX/dt constant) where the water transport is limited by the evaporation of the water accumulated at the surface and a falling-rate (FR) period (|dX/dt| decreasing with time) where the water transport is limited by the diffusion from the interior of the solid to the surface. We now address the different drying behaviors shown in Figure 5. The transitional (TR) period can be regarded as an adjustment phase during which the temperature of the sample is adjusted to its final value. It has a short duration (approximately 6 h) compared to the total duration of the drying. During this period, the drying curves of the three gels have very similar shapes and durations with the drying limited by the transport of water from the interior of the gel to the

Figure 4. AFM images (2 × 2 μm2) of (left-hand pictures) the dehydrated sugar-free gel and (right-hand pictures) the dehydrated gel with 15 wt % glucose; the upper pictures are topology images, whereas the lower ones are phase images (glucose, white/light brown; silica, dark brown).

sugar-free gel structure observed macroscopically in Figure 1. Measurements of the topology and the phase of the tip as a function of position provide complementary images of the probed surface: the topology image contains information about the surface 3D structure, whereas the phase image discriminates between its different components. The topology images (Figure 4) show that the sugar-free gel is composed of a dense packing of primary silica particles forming large aggregates (∼100 nm), whereas that with 15 wt 9484



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Figure 5. Experimental drying curves for various sugar contents ϕ (wt %). The drying rate is defined as −dX/dt and the moisture ratio as M(t) = X(t)/X(0). Symbols of the same type represent measurements at the same elapsed time: ● (7 h), ★ (27 h), ▲ (58 h). The labels TR, Q-CR, FR1, and FR2 represent the periods with different drying behavior described in the text.

medium. Assuming that the water transport obeys Fick’s law and that the diffusion of water within the gel can be considered as one-dimensional, which should be valid since the gel is firmly attached to the side walls of the container and so the sides can be considered impermeable, the water concentration C(z,t) in the gel obeys the equation:

exchange surface. As long as the nature of the gel does not change significantly in this period − as implied by the photographs in Figure 1 − the porosity is either constant or varies slightly but in the same way for the three gels. Ongoing rheological measurements show that the dynamic viscosities of water−sugar solutions with compositions similar to those used here are like that of pure water. The dynamic viscosities of the solutions in the porous matrix remain very close to that of water, so the porous network does not significantly affect the water transport. The second period is a quasi-constant-rate (Q-CR) period during which the drying rate −dX/dt is almost constant and independent of the water content. As in the period TR, the curves for the three gels have the same shape and same duration (20 h). Here, it is the evaporation of water from the exchange surface that limits the drying dynamics, independent of the water content, provided that water not tightly bound to the porous network is available. The results in periods TR and Q-CR show that, at moisture ratio higher than 0.9, the sugar does not have a significant influence on the water mobility in the porous matrix. In period Q-CR, however, the drying rate decreases with increasing sugar concentration, which can be ascribed to the hydrogen bonds between the water and sugar molecules, requiring more energy to extract the water: glucose is now playing a protective role. When all the water not tightly bound to the porous network has evaporated, we find a falling-rate period FR1 where drying is limited by the circulation of water within the porous network. In the samples with low sugar concentration, internal deformations of the porous network begin to appear due to an increase in capillary pressure. These deformations produce a recirculation of the water bound to the network, evaporating as it is released. This progressive release of water limits the drying kinetics during this period. For the gel with the highest sugar content (30%), the drying is arrested at the end of the period, maintaining the moisture ratio at a constant value near 0.8 for more than 200 h. This gel does not undergo any deformation, as shown by the photographs in Figure 1. By strongly slowing down the evaporation kinetics of the bound water, the sugar limits the increase in capillary pressure that would otherwise contract the network of capillaries constituting the porous

∂C(z , t ) ∂ ⎛ ∂C(z , t ) ⎞ = ⎜Da ⎟ ∂t ∂z ⎝ ∂z ⎠

(3)

Here, z is the diffusion length referenced from the bottom surface of the sample and Da an apparent diffusion coefficient because the water transport through the porous matrix during this phase involves diffusion of both vapor and liquid as well as the internal evaporation and surface evaporation. Solving eq 3 with boundary conditions corresponding to an isothermal vacuum drying from one side only, without shrinking, we obtain the following relation:33 X (t ) =

8X(0) π2

∞

∑ n=0

⎛ (2n + 1)2 π 2D ⎞ 1 a ⎜− exp t⎟ (2n + 1)2 4L2 ⎝ ⎠ (4)

where L is the sample thickness. Fits of eq 4 to the experimental data yield the experimental values of Da shown in Table 1. The values for the gels with 0 and 15 wt % sugar concentration are almost the same, but at 30 wt % sugar the apparent diffusion is much slower. It should be noted that these values are lower than those obtained from quasi-elastic neutron scattering data.2,34 The final drying period FR2 exhibits the most complex behavior because the deformations of the porous network are Table 1. Values of the Apparent Diffusion Constant Da in Period FR1; L is the Sample Thickness and R2 the Coefficient of Determination

9485

glucose concentration (wt %)

0

15

30

L (cm) Da (m2.s−1) R2

2.0 4.3 × 10−11 0.97

2.5 4.2 × 10−11 0.98

3.0 1.5 × 10−11 0.95



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(9) Sum, A. K.; Faller, R.; de Pablo, J. J. Biophys. J. 2003, 85, 2830− 2844. (10) Patist, A.; Zoerb, H. Colloids Surf. B 2005, 40, 107−113. (11) Miller, D. P.; de Pablo, J. J. J. Phys. Chem. B 2000, 104, 8876− 8883. (12) Ellis, R. J.; Minton, A. P. Nature 2003, 425, 27−28. (13) Ball, P. Chem. Rev. 2008, 108, 74−108. (14) Brook, M. A.; Chen, Y.; Zhang, Z.; Brennan, J. D. J. Mater. Chem. 2004, 14, 1469−1479. (15) Chen, Y.; Zhang, Z.; Sui, X.; Brennan, J. D.; Brook, M. A. J. Mater. Chem. 2005, 15, 3132−3141. (16) Wei, Y.; Jin, D.; Ding, T.; Shih, W.-H.; Liu, X.; Cheng, S. Z.; Fu, Q. Adv. Mater. 1998, 3 (4), 313−316. (17) Wei, Y.; Xu, J.; Dong, H.; Dong, J. H.; Qiu, K.; Jansen-Varnum, S. A. Chem. Mater. 1999, 11, 2023−2029. (18) Lelong, G.; Price, D. L.; Douy, A.; Kline, S.; Brady, J. W.; Saboungi, M.-L. J. Chem. Phys. 2005, 122, 164504. (19) Lelong, G.; Price, D. L.; Brady, J. W.; Saboungi, M.-L. J. Chem. Phys. 2007, 127, 065102. (20) Jasnin, M.; Moulin, M.; Haertlein, M.; Zaccai, G.; Tehei, M. EMBO Reports 2008, 9, 543−547. (21) Persson, E.; Halle, B. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 6266−6271. (22) Jaymes, I.; Douy, A.; Massiot, D.; Busnel, J.-P. J. Am. Ceram. Soc. 1995, 78, 2648−2654. (23) Brinker, C. J.; Scherer, G.W. In Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press, 1990. (24) Keiderling, U. Appl. Phys. A: Mater. Sci. Process 2002, 74, S1455−S1457. (25) Kilburn, D.; Claude, J.; Schweizer, T.; Alam, A.; Ubbink, J. Biomacromolecules 2005, 6, 864−879. (26) Kilburn, D.; Townrow, S.; Meunier, V.; Richardson, R.; Alam, A.; Ubbink, J. Nat. Mater. 2006, 5, 632−635. (27) Kilburn, D.; Claude, J.; Mezzenga, R.; Dlubek, G.; Alam, A.; Ubbink, J. J. Phys. Chem. B 2004, 108, 12436−12441. (28) Hoinkis, E. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Vol. 25, Marcel Dekker, 1997. (29) Lelong, G.; Price, D.L.; Saboungi, M.-L. Nanoporous Materials: Advanced Techniques for Characterization, Modeling, and Processing/ Chapter 1: Scattering Techniques; Taylor & Francis, 2011; -ISBN: 9781-4398110-4-7. (30) Wei, Y.; Xu, J. G.; Dong, H.; Dong, J. H.; Qiu, K. Y.; JansenVarnum, S. A. Chem. Mater. 1999, 11, 2023−2029. (31) Bhattacharyya, S.; Lelong, G.; Saboungi, M.-L. J. Exp. Nanoscience 2006, 1 (3), 375−395. (32) ″Drying of Foods, Vegetables and Fruits″, Vol 1, 2010, Editors: Sachin V. Jangam, Chung Lim Law Arun S. Mujumdar. ISBN: 978981-08-6759-1 (33) Crank, J. In The Mathematics of Diffusion; Oxford Univ. Press: London, 1975. (34) Lelong, G.; Howells, W. S.; Brady, J. W.; Talon, C.; Price, D. L.; Saboungi, M.-L. J. Phys. Chem. B 2009, 113, 13079−13085.

now very large and generate recirculation of the bound water, even in the gel with the highest sugar content. The gel without sugar is deformed in an irreversible fashion with the appearance of small pebbles of silica that lock up less than 1% of the water content whereas after the same elapsed time (1300 h) the gel with 15 wt % glucose contains nearly 4% water content and that with 30 wt % sugar 10% water content.

5. CONCLUSIONS The combination of macroscopic measurements (gravimetry and optical photography) with microscopic probes utilizing both diffraction (SANS) and imaging (AFM) techniques leads to a consistent and compelling picture of the role played by simple sugars in the stabilization of silica gels and, by inference, biological organisms against drastic dehydration from the macroscopic to the microscopic scale. The presence of glucose leads to a remarkable improvement in the resistance to dehydration. SANS spectra confirmed that the structural integrity of the gel is maintained when 30 wt % of glucose is added. The AFM results show that sugar is well dispersed in the silica matrix. The reaction-limiting factor appears to be the siliceous backbone, which, at the same time, is prevented from fracturing by the presence of sugar. The sugar−silica affinity provides the stabilization of the gel upon the addition of substantial quantities of the sugar. The gravimetric analysis indicates that the gels with different sugar content have a similar behavior during the entire vacuum drying process. The main role played by the sugar is to slow down the evaporation kinetics of the water bound to the network becoming more significant as the sugar concentration increases. By limiting the evaporation of bound water, the sugar hinders the progressive increase in capillary pressure that leads to internal stresses deforming the porous network and generating considerable and irreversible modification of its structure.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the “Inside Pores” European Network of Excellence and by Agence Nationale de la Recherche (BIOSTAB program). G.L. acknowledges the Region Centre for a SOLEIL scholarship. We thank A. Lerbret and J. Teixeira for helpful discussions and the HelmholtzZentrum Berlin for providing the neutron scattering facilities.

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REFERENCES

(1) Franks, F. In Biophysics and Biochemistry at Low Temperatures; Cambridge Univ. Press: Cambridge, 1985. (2) Wolfe, J.; Bryant, G. Cryobiology 1999, 39, 103−129. (3) Crowe, L. M. Comparative Biochem. Physiol. Part A 2002, 131, 505−513. (4) Holmström, K.-O.; Welin, E. M. B.; Palva, A. M. E. T.; Tunnela, O. E.; Londesborough, J. Nature 1996, 379, 683−684. (5) Cesàro, A. Nat. Mater. 2006, 5, 593−594. (6) Hincha, D. K.; Hagemann, M. Biochem. J. 2004, 383, 277−283. (7) Green, J. L.; Angell, C. A. J. Phys. Chem. 1989, 93, 2880−2882. (8) Crowe, J. H.; Carpenter, J. F.; Crowe, L. M. Annu. Rev. Physiol. 1998, 60, 73−103. 9486


Role of Glucose in Enhancing Stability of Aqueous Silica Gels Against

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