def _find_in_smiles(idx, reactants):
    """
    Find atom label in list of reactant SMILES.
    Input:  idx: atom index.
            reactants: list of reactant SMILES.
    Output: reactant: SMILES of reactant that has the indicated atom.
    """
    search = ':' + idx + ']'
    for reactant in reactants:
        if search in reactant:
            return reactant
