J O U R N A L O F C H E M I S T R Y Materials Preparation of 2,5-diphenyloxazole doped sol-gel glasses and their application to radio-analytical chemistry Ian Hamerton, John N. Hay, John R. Jones* and Shui-Yu Lu Department of Chemistry, School of Physical Sciences, University of Surrey, Guildford, Surrey, UK GU2 5XH Received 5th June 1998, Accepted 9th September 1998 Transparent, colourless, monolithic sol-gel glasses doped with 2,5-diphenyloxazole (PPO) at diVerent concentrations were prepared via the polymerisation of tetraethyl orthosilicate (TEOS) in a H2O–AcOH–EtOH–PPO solution.The gel samples were aged at room temperature, and dried in two stages: first at room temperature, then at 70 °C to reduce possible fracture. They have pore diameters in the range 1.8–2.1 nm.The leaching of PPO from the sol-gel glass was monitored using ultraviolet (UV) and X-ray photoelectron spectroscopy (XPS). The results show that the sol-gel glasses dried at the higher temperature leach less than those dried at room temperature. These sol-gel glasses, either in monolith or powder form, are eVective in detecting b- radioactivity. should be kept to a minimum.Hence, a counting method Introduction requiring minimum use of solvent and recycling of the primary/ The development of liquid scintillation counting in the early secondary solute would represent a major advance in radio- 1960s has had a dramatic eVect on the use of radioisotopes, analytical chemistry technology. particularly in the physical and life sciences.1,2 The main The sol-gel method is a convenient low temperature route reason for this is that the two most widely used radioisotopes to converting metal alkoxides into the corresponding inorganic in this area, 3H and 14C, are both weak b- emitters, so that gels and glasses under relative mild conditions.6 An approother counting techniques such as ionisation, Geiger–Mu� ller priate precursor, such as tetraethyl orthosilicate (TEOS) and proportional counting are either impractical or have undergoes acid catalysed hydrolysis [eqn.(1)], followed by serious limitations. The advantages of liquid scintillation condensation polymerisation [eqn. (2)] at low temperature in counting for weak b- emitters are many: (a) high eYciency, a suitable solvent to form the polymeric silica network.typically >50% for 3H and >90% for 14C; (b) high sample Si(OEt)4+4H2O�Si(OH)4+4 EtOH (1) throughput as instrumentation is easily automated; and (c) a wide range of samples, from simple organic to complex n Si(OH)4�n SiO2+2n H2O (2) biological, can be analysed.3,4 The low temperature provides a clean route for doping The need for further improvement stems from the general inorganic gels and glasses with organic molecules, and in the tightening in radioactive waste legislation that has been in process makes it possible to develop composite materials with evidence over the last decade.5 A liquid scintillator consists of specific properties, and hence opens the way to various potena solvent, a primary solute (e.g. 2,5-diphenyloxazole) and, tial applications.For example, organic doped sol-gel glasses, sometimes, a secondary solute {e.g. 1,4-di[2-(5-phenyl- ranging from monoliths to thin films, have been developed as sensors for metal cations,7–10 protons (H+),9–13 anions,14,15 neutral species,16–19 oxygen20 or carbon monoxide14 in water, and oxygen21 or carbon monoxide21,22 in the gas phase. We sought to incorporate PPO into a sol-gel silica monolith in order to exploit fully its eventual sensing properties in radio-analytical chemistry technology.The chemical and physical properties of the PPO are hopefully retained, whilst the pore network allows external molecules, tritiated water (HTO) in this case, to diVuse into the matrix and interact with the solute. In this paper we report a preliminary investigation into the preparation and characterisation of sol-gel glasses doped with PPO at diVerent concentrations.Pore size, surface area and leaching tests and application in detecting 3H radioactivity are of particular interest. O N N O O N POPOP PPO Experimental oxazolyl )benzene], POPOP}. The radioactive sample is either dissolved directly in the scintillator, or if insoluble, through Materials the addition of a blending agent or secondary solvent— TEOS (Aldrich, 98%), TMOS (Acros, 99%), glacial acetic acid dioxane is frequently used for this purpose.Cocktails have (Fisons, Anal.), absolute alcohol 100 (Hayman Ltd., 99.86%), been developed so as to maximise the amount of aqueous PPO (Acros, scintillation grade), toluene (Fisons, low in samples that can be dissolved in the scintillator.3,4 After sulfur, Anal.) were used as received.counting it is customary practice to dispose of the radioactivity via the drains or absorb it onto vermiculite. Large numbers of Instrumentation samples can generate a considerable volume of waste. Ideally the radioactive waste should be stored in as compact a form BET surface areas and pore sizes were measured on a Coulter SA3100 Surface Area and Pore Size Analyser.The samples as possible and the work necessary to bring it to this state J. Mater. Chem., 1998, 8(11), 2429–2432 2429Table 1 Preparation of PPO doped sol-gel glassesa were outgassed at 120 °C for 240 min. UV spectra were recorded using a Philips PU8740 UV/VIS Scanning Composition of each vial/mmol Colourless Spectrophotometer. Liquid scintillation counting was carried Fracture- monolith out using a Packard Tri-Carb 1500 Liquid Scintillation No.b TEOS EtOH H2O AcOH PPO free gel? upon drying? Analyser, toluene was the preferred solvent.XPS measure- 1 4.58 13.73 9.15 0.35 0.000 Yes Yes ments were made using a VG Scientific ESCALAB Mk II 2 4.58 13.73 9.15 0.35 0.014 Yes Yes Spectrometer interfaced with a VG500S data system based on 3 4.58 13.73 9.15 0.35 0.027 Yes Yes a DECPDP 11/73 computer.The operating conditions were 4 4.58 13.73 9.15 0.35 0.054 Yes Yes as follows: the X-ray source (Mg-Ka 1253.6 eV radiation) was aDuplicates of 10 vials were prepared. The mixture was acidic (pH operated at a power of 450W (i.e. 13 kV potential and 34 mA 3.5). Gelation and aging were carried out at room temperature.The emission current). The spectrometer was operated in the fixed final drying temperature was 70 °C. bThe same numbering system was analyser transmission mode at a pass energy of 50 eV. The used throughout Table 1–5. Each number refers to the monolithic base pressure in the sample chamber during analysis was sol-gel glasses which were prepared in the same batch with identical PPO concentration.approximately 3×10-8 mbar. The sample was kept in the preparation chamber under vacuum overnight. Preparation of PPO doped sol-gel monolith ture with around 5% (w/w) weight loss per day for one week. Fracture-free, transparent and disc-shaped sol-gel glasses were Typically a mixture of TEOS (10.2 cm3, 45.8 mmol), water obtained after further drying at 70 °C overnight.The tempera- (1.65 cm3, 91.5 mmol), ethanol (8.1 cm3, 137.3 mmol), acetic ture is just slightly below the melting point of PPO (72–74 °C). acid (0.2 cm3) and PPO (amount appropriate for concentration All operations were carried out under normal atmosphere. required) was obtained as a clear, colourless solution. A portion of the solution (2.0 cm3) was sealed in a glass vial Surface area and pore size (20 cm3).Usually 10 such vials were prepared for one particular PPO concentration at a time. These vials were then kept Surface area and pore size data are summarised in Table 2. at room temperature to gel (1 week) and age (1 week) under The average pore diameter (d) is estimated from d=4V/S, normal atmosphere. The gels were then allowed to dry at where V is the total pore volume and S is the surface area.room temperature for one week, followed by drying at 70 °C The Coulter SA3100 Analyser uses the gas sorption method; overnight. Sol-gel glasses were obtained as clear, colourless the inert gas adsorbate is nitrogen. The BET (Brunauer, and fracture-free monoliths in cylindrical disc form. Emmett, Teller) calculation is used for the determinof the sample specific surface area.The BJH (Barrett, Joyner, Leaching test Halenda) calculation yields the pore size distribution. Coulter VacJack sample tubes, which minimise the eVects of changing Leaching tests were carried out using UV and XPS methods. liquid cryogen level as cryogen boils away during long pore For UV monitoring, sol-gel glass was ground to a powder parameter analysis, were used.By using these sample tubes form. To an UV cell (quartz, 1 cm) filled with toluene (3 cm3), BET surface area reproducibility was better than ±2%.23 was added a known amount of sol-gel glass sample. The These PPO doped sol-gel samples have very similar surface mixture was shaken at regular intervals to be mixed thorareas.PPO concentration (up to 2×10-2 M) did not seem to oughly. UV spectra were recorded at various time intervals to aVect the surface area or average pore diameter. obtain the profiles of PPO concentration in the solution, while the solids separated from the solution and settled at the bottom Leaching of the cell. For XPS analysis, four sol-gel glass discs, representing PPO concentrations at 0, 1, 2 and 4 g dm-3, were PPO was reported to display a high intensity absorption band used without further treatment.Another four sol-gel glass in the 300–335 nm region due to the p–p* through conjugate discs were packed in a small column, separated by filter papers transition in the aromatic structure. Fine details of this absorpand thoroughly washed by a constant toluene stream that was tion band are recognisable in cyclohexane solution, with peaks maintained at a rate of 2 cm3 min-1.The sol-gel glasses were at 302 and 318 nm, plus shoulders at 310 and 333 nm.24 In then dried at 70 °C overnight and then mounted on the XPS toluene, PPO (9×10-3 M) also displays a very similar absorpsample stand using ‘super glue’ for analysis.tion band with peaks at 307 and 319 nm, plus shoulders at 311 and 335 nm. Hyperchromic eVects were observed when Liquid scintillation counting procedure toluene was replaced by EtOH or TEOS as solvent. However, as observed in the BET surface analysis experiments, sol-gel To a micro-vial (0.5 cm3) was added stock HTO solution in glasses dried at 70 °C only lose a further 5% weight when 1,4-dioxane, toluene (0.4 cm3) and sol-gel glass powder subjected to heat treatment at 120 °C for 4 h.This means that (0.05–0.2 g). The micro-vial was then inserted into a standard the level of any volatile residue, such as solvent (EtOH), glass scintillation vial (25 cm3), and counted for 5 min for catalyst (AcOH) or unreacted starting materials (water, good statistical disintegration per minute (DPM) values.TEOS) will not exceed 5% in weight. A 100 mg sol-gel glass sample can release ca. 5 mg impurity to the 3 cm3 of toluene Results and discussion in a UV cell. UV absorptions of PPO in toluene spiked with 0.2% (v/v) impurity such as EtOH, AcOH or TEOS were all Sol-gel preparation identical. The cut-oV wavelength for toluene is 285 nm25 and Both TMOS and TEOS were used in trials to prepare PPO doped sol-gel glasses.At room temperature, gelation for both Table 2 Surface area and pore size measurements HCl catalysed systems was slow. Acetic acid catalysed systems gave fracture-free gels. Without catalyst, precipitation of white No. Drying temp./ °C Surface area S/m2 g-1 Average pore solids was observed in the TEOS system. We therefore decided diameter d/A° to use the system containing TEOS–EtOH–H2O–AcOH (see 1 70 610 21 Table 1).To avoid fracture, the gels were aged at room 2 70 540 20 temperature for a week, whilst the vials containing the gel 3 70 506 20 were kept tightly closed to avoid the loss of solvents. After 4 70 515 21 ageing the gels were dried in two stages: first at room tempera- 2430 J.Mater. Chem., 1998, 8(11), 2429–2432Table 4 Counting eYciency for PPO doped sol-gel glasses in EtOH–toluene mixture Sol-gel HTO Radioactivity/ Relative No. glass/g sample/g dpm g-1 eYciency (%)a 1 0.076 0.3616 22904 74.6 2 0.059 0.3602 25318 82.5 3 0.057 0.2608 26863 87.5 4 0.060 0.3545 27367 89.2 aCompared to a toluene-based scintillator (3.4 g dm-3 PPO). Standard HTO radioactivity=30694 dpm g-1.Table 5 Counting eYciency for PPO doped sol-gel glasses after toluene wash in EtOH–toluene mixture Sol-gel HTO Radioactivity/ Relative No. glass/g sample/g dpm g-1 eYciency (%)a 1 0.125 0.4116 60409 66.8 2 0.104 0.4130 65965 73.0 3 0.111 0.4139 59698 66.0 4 0.059 0.4115 70140 77.6 aCompared to a toluene-based scintillator (3.4 g dm-3 PPO). Standard HTO radioactivity=90408 dpm g-1.of PPO. This observation also supports the UV results that the PPO concentration increased in the toluene solution due to leaching at the sol-gel surface or outer layers. However, PPO was also encapsulated in the inner pores, and suYcient concentration was retained after the toluene treatment. These sol-gel glasses still gave more than 60% relative eYciency when used in liquid scintillation counting (see Table 5).Liquid scintillation counting using PPO doped sol-gel glasses The eYciency of PPO doped sol-gel glasses to detect b- radioactivity (from tritiated water, HTO) was investigated using a liquid scintillation counting method. Preliminary Fig. 1 Changes in UV spectra, representing PPO leaching from results are shown in Table 4 and 5, and the counting eYciency (A) sol-gel glass (No. 4 in Table 1) dried at room temperature and was compared with the standard PPO solution in toluene as (B) sol-gel glass (No. 4 in Table 2) dried at 70 °C. These spectra were scintillator. When sol-gel glasses were used with toluene alone, recorded at (a) 0, (b) 2, (c) 5, (d) 10, (e) 20, ( f ) 30 and (g) 60min after mixing the sol-gel glass with toluene in a UV cell.the relative eYciency was very low, possibly due to some preferential absorption of water molecules onto the glass. It is also possible that hydrogen–tritium (H–T) exchange takes the main absorption band for PPO is in the 300–335 nm place on the surface Si–OH groups.26 This can gave rise to region. With these factors in mind, together with the advantage chemical quenching because at this stage it appears that the of monitoring the leaching in toluene which is the major PPO released from the freshly broken surface was the eVective scintillation solvent, the choice of toluene as solvent was fluor.The energy of radiation was not freely passed between entirely satisfactory. the solvent and fluor molecules, thus fewer photons were UV spectra representing the PPO concentrations in toluene generated.However, with careful adjustment of the scintil- are shown in Fig. 1. The increase of PPO concentration reflects lation cocktail, in this case by adding EtOH to suppress the the leaching from sol-gel glasses in a semi-quantitative manner. surface H–T exchange or preferential absorption, high The rate of leaching was greater in the earlier stage of the eYciency, typically >60%, can still be achieved.The liquid mixing, then decreased gradually for sol-gel glasses dried at scintillation counting method provides important clues about room temperature [Fig. 1(A)]. Sol-gel glasses treated at higher the eVectiveness of sol-gel encapsulation. Furthermore it may temperatures should have shrunk structures, and tend to leach yield more information about the mechanistic details as to less than their room temperature dried counterparts during how radioactivity interacts with the solvent and fluor within/ the same period [Fig. 1(B)]. XPS analysis (Table 3) of the outside the sol-gel cages. 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