## 1 Introduction  
Colloidal gold nanoparticle (Au NP) solutions used in biological assays need to be stable over a wide range of ionic strengths. Therefore, it is essential that attention is paid to the choice of the capping ligands, as these surface bound ligands are responsible for stabilizing NPs against aggregation and providing the desired NP properties (e.g., hydrophilicity/hydrophobicity, surface charge, ligand arrangement, chemical reactivity) [1; 2]. A general method of stabilizing Au NPs is the use of electrostatic repulsion between Au NPs, which are modified with charged ligands such as citrate [3] or triphenylphosphine [4]. However, the stability of electrostatically stabilized Au NPs is affected by the solution properties, including pH, ionic strength, and temperature [5]. Several approaches have been developed in order to improve the stability of Au NPs under biological conditions and these generally involve the functionalization of Au NPs with ligands that provide either steric or electrostatic stabilization, thus imparting stability to the Au NP colloid. One of the most commonly used approaches involves the modification of the Au NP surface by assembling a monolayer of peptide-capping ligands [6]. A combinatorial design approach enables the synthesis of ligands that can enhance the stability of the Au NPs in aqueous media [7]. The design strategy of these peptides takes into account the ability of certain amino acid residues to self-assemble into a dense layer that excludes water and a hydrophilic terminus to ensure solubility and stability in water [8]. The aim of this design approach is to produce ligands that, coupled to the Au NPs, will ensure the ligand is bound to the particles through the thiol group on the terminal cysteine. A different widely applied method is the incorporation of a thiol-containing polyethylene glycol (dithiol-PEG) moiety into the functional ligand, because of its good solubility, biocompatibility, and anti-fouling properties, which render NPs resistant to non-specific protein adsorption [9; 10]. A number of workers have utilized functional PEGylated Au NPs in targeted drug delivery and as biosensors in complex media [11; 12; 13]. [PERSON] and colleagues investigated the effect of using multiple thiol linker systems on minimizing undesirable aggregation events caused by thiol desorption, resulting in the loss of function of the probe [14]. Dithiols have also been used because of their higher coupling efficiency to Au NP surfaces, compared to monothiol, which thus enhances the monolayer stability [15; 16]. A number of dithiols such as dihydrolipoic acid derivatives [9; 17] and di- or polythiol PEGs [18; 19] have been assessed for the functionalization of Au NPs in order to impart further stability. In this report, the effect of incorporating different stabilizer moieties into the assay format was tested for the colorimetric detection of the DPP-IV enzyme (Scheme 1) --a dipeptidase enzyme of high clinical importance, especially as a diagnostic and prognostic marker of various inflammatory disorders, tumors, and hematological malignancies [20; 21]--by three distinct methods and was assessed in order to establish the most stable platform that could be exploited for use in high-ionic-strength media such as biological fluids. Table 1 displays the complete sequence of the peptide constructs used in these experiments. **Scheme 1. ** The mechanism of change of color of the particles. Structure of functional and stabilizer-ligands. **(A)** C/G dipeptide Au NPs, **(B)** GPGCALNNC and **(C)** GPG-EN-PEG4-LA. These approaches were chosen because they are well established in their use in NP systems and they have predictable stability properties [5]. The aim of this study was to enhance the stability of functionalized Au NPs by incorporating stabilizers and to evaluate their effect on DPP-IV detection.