NOVELTY - A nanoparticle/dispersant complex comprises a nanoparticle covered with at least one dispersant (I).    USE - As nanoparticle/dispersant complex useful in a nanoparticle dispersion liquid; and in a nanoparticle/matrix-material complex (claimed) useful for applications including as optical material, liquid material having high refractive index, photonic crystal material, dielectric material, electrode material, electronic/semiconductor material, solar battery material, cosmetic material, coloring material, painting material, coating material, and as a catalyst material.    ADVANTAGE - The nanoparticle/dispersant complex has excellent dispersibility and long-term stability in a dispersion medium, and nanoparticle dispersion liquid and nanoparticle/matrix-material complex comprising the nanoparticle/dispersant complex are colorless and transparent even at high concentrations. Moreover, the nanoparticle/dispersant complex can be produced easily.    DETAILED DESCRIPTION - A nanoparticle/dispersant complex comprises a nanoparticle covered with at least one dispersant (I) of formula R1-A+-R2-Y'- (Ia) or R1-A+-R2-Y'H.X- (Ib).    A+=a heterocyclic cationic group;    R1=H or an optionally substituted 1-30C alkyl;    R2=a single bond or an optionally substituted 1-30C alkylene;    Y'-=an anion moiety of an oxo acid group containing a sulfur atom or a phosphorus atom;    Y'H=an oxo acid group containing a sulfur atom or a phosphorus atom;and    X-=an anion produced by dissociation of a proton from a protonic acid.    INDEPENDENT CLAIMS are included for the following:    (1) a nanoparticle dispersion liquid, where a nanoparticle/dispersant complex is dispersed into a dispersion medium;    (2) a nanoparticle/matrix-material complex comprising a nanoparticle/dispersant complex that is dispersed in a matrix material;    (3) producing a nanoparticle/dispersant complex involving forming under a presence of at least one dispersant (Ia) or (Ib), a nanoparticle covered with the dispersant from a nanoparticle precursor; or bringing a nanoparticle and at least one dispersant (Ia) or (Ib) into contact with each other so that the nanoparticle is covered with the dispersant;    (4) a new phosphorus compound containing a nitrogen-containing aromatic heterocyclic cationic group represented by the formula R1-Ar+-R2-B1-PO(OR3)(OR4).X- (III); and    (5) method for producing the phosphorus compound containing a nitrogen-containing aromatic heterocyclic cationic group (III).    Ar+=a nitrogen-containing aromatic heterocyclic cationic group;    B1=a single bond or an oxygen atom;and    R3 and R4=H or an optionally substituted 1-10C alkyl.