chloride dissolved in 1 6 ml deionized water was used as the aqueous core precursor 191 mg sodium borohydride was dissolved in 1 5 ml of deionized water for use as the reducing agent the surfactant solution was prepared using 28 0 grams cetyltrimethylammonium bromide ctab dissolved in 200 ml of chloroform the aqueous metal solution was mixed with 50 ml ctba solution and placed in a flask under flowing nitrogen the sodium borohydride solution was mixed with 50 ml of the ctab solution and sonicated for four minutes to degas and homogenize the sodium borohydride ctab solution was added to the iron chloride ctab solution and allowed to react with magnetic stirring under flowing nitrogen for 45 minutes the shell precursor was prepared using 210 mg of chromium ii chloride mixed with 1 8 ml deionized water the solution was sonicated for one minute and centrifuged at 5000 rpm for five minutes the solution was decanted into 50 ml ctab solution and sonicated for 10 minutes additional 150 mg of sodium borohydride was dissolved in 1 8 ml of deionized water and added to 50 ml ctab solution the micelle metal solution for forming the shell was injected into the reaction vessel containing the core material as described in the immediately preceding paragraph the reaction was allowed to react for five minutes the reaction solution was quenched by adding a large excess of chloroform.