Water-soluble polymer and water-soluble nanoparticle composite

A water-soluble polymer, comprising:

• a repeating unit represented by Formula (1):
• (wherein R1 is —COOH, —OH, or —(OCH2CH2)pR′ (wherein R′ is —COOH, —OH, —CH═CH2, —C≡CH, or —N3, and p is an integer from 1 to 30.), and 1 is an integer from 1 to 10.),
• a repeating unit represented by Formula (2):
• (wherein R2 is —SR3 or —NHR4 (wherein R3 is hydrogen or a thiol-protecting group selected from the group consisting of benzyl, methoxybenzyl, N-(acetyl)aminomethyl, t-butyl, methylbenzyl, 3,4-dimethylbenzyl, triphenylmethyl, benzhydryl, acetamidomethyl, carbomethoxysulfenyl, 3-nitro-2-pyridinesulfenyl, ethylcarbamoyl, 9-fluorenylmethyl, and pyridyl sulfide, and R4 is hydrogen or an amino-protecting group selected from the group consisting of tert-butoxycarbonyl, benzyloxycarbonyl, 9-fluorenylmethyloxycarbonyl, 2,2,2-trichloroethoxycarbonyl, allyloxycarbonyl, phthaloyl, p-toluenesulfonyl, and 2-nitrobenzenesulfonyl);
• a, the number of repeating units containing —SR3, and b, the number of repeating units containing —NHR4, in y, the number of repeating units, satisfy b/a+b=0 to 0.60;
• and m and n are an integer from 1 to 10.), and
• a repeating unit represented by Formula (3):
• wherein x, the number of repeating units of Formula (1), y, the number of repeating units of Formula (2), and z, the number of repeating units of said Formula (3), satisfy x/(x+y+z)=0.01 to 0.70, y/(x+y+z)=0.20 to 0.70, and z/(x+y+z)=0.01 to 0.70.