Determination of GaN nanosensor for scavenging of toxic heavy metal ions (Mn<SUP>2+</SUP>, Zn<SUP>2+</SUP>, Ag<SUP>+</SUP>, Au<SUP>3+</SUP>, Al<SUP>3+</SUP>, Sn<SUP>2+</SUP>) from water: Application of green sustainable materials by molecular modeling approach