TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Dimensions: The soluble nanoparticles preferably have a diameter of 0.5-3 (preferably 1-2) nm.Preferred Method: The soluble nanoparticles are made e.g. by reacting a Pt compound alone or a Pt compound together with one or more compounds of rhodium, ruthenium, iridium or palladium with a reducing agent in water or a solvent, and either carrying out the reduction in the presence of a polymeric betaine or adding the polymeric betaine to the solution after the reduction step. The solution may be purified by precipitation and/or concentrated by evaporation. Suitable starting materials for preparing the soluble nanoparticles are soluble (especially water-soluble) salts such as hexachloroplatinic(IV) acid hydrate, hexachloroiridium(IV) acid hydrate, palladium(II) acetate, iridium(III) acetylacetonate, ruthenium(III) acetylacetonate, ruthenium(III) nitrate or rhodium(III) chloride hydrate. Suitable reducing agents are H2, NaH, ethanol, ethylene glycol or (especially) hydroxymethanesulfinic acid sodium salt. The reducing agent is used in an amount of from the stoichiometric amount to a 10-100 % excess with respect to the metal compound.    TECHNOLOGY FOCUS - POLYMERS - Preferred Compounds: The protective colloid comprises at least one polymer with betaine groups and having a side chain consisting of a 2-12 C (preferably 2-4 C) alkylene group.The betaine is e.g. a carbobetaine of formula -N+(R1)(R2)-(CH2)N-CO2-, a phosphobetaine of formula -N+(R1)(R2)-(CH2)n-PO3- or (especially) a sulfobetaine of formula -N+(R1)(R2)-(CH2)n-SO3-.R1 and R2 = 1-6 C alkyl; andn = 1-3.The protective colloid preferably contains compounds of formula (I), (II) or (III):n = 1-3.The polymer with betaine groups has a degree of polymerization of 50-10000, especially 100-1000. The polymer may contain, in addition to monomers containing betaine groups, comonomers selected from acrylic acid, acrylic esters, acrylamide, carboxylic acid vinyl esters, vinyl-alkylethers, N-vinylpyridine, N-vinylpyrrolidone, or N-vinylcarboxylic acid amides. EXAMPLE - Hexachloroplatinic acid hydrate (2.50 g) was added to 1000 ml deionized water, then 5 % ammonia solution was added drop wise to a pH of 7. The solution was stirred at 95degreesC and Poly-(N,N-dimethyl-N-methacryloxyethyl-N-(3-sulfopropyl)-ammonium betaine) (formula (I); 1.0 g) was added followed by a solution of hydroxymethanesulfinic acid sodium salt dihydrate (2.50 g) in water (20 ml). The mixture was allowed to stand and to cool to room temperature for 8 hours, then acetone (1000 ml) was added to the hydrosol, stirred for 5 minutes and allowed to stand for 15 hours to allow the precipitate to settle out. The product was recovered by decanting and centrifuging; the centrifuged residue was dissolved without residue in water (20.0 g) to yield a dark red-brown solution.