## 3 Results and Discussion  
The well-known reduction of tetrachloroauric acid with ascorbic acid in aqueous solution at room temperature leads to gold nanoparticles between about 10 and 20 nm [41; 42]. The presence of the polycationic macromolecule poly-DADMAC leads to particles of different characters and aggregations. Typically, polynuclear compact elementary particles of about 10 nm diameter are observed, but the in situ assembly behavior is dependent on the poly-DADMAC concentration. Upon the reduction of 50 nmol HAuCl4 with 100 nmol ascorbic acid in 1 mL aqueous solution (Figure 1), the aggregation tendency of the elementary particles is already visible at a poly-DADMAC concentration of 8 ppm, leading to compact aggregate particles of about 75 nm. These particles sometimes aggregate further to form larger compact particle clusters (Figure 1h). Typically, a mixture of small particles (between about 6 and 30 nm), medium-sized particles of about 80 nm and aggregates in the submicron range is formed (Figure 1g). In contrast, characteristic dendritic-shaped clusters of small gold nanoparticles are found at a poly-DADMAC concentration of 60 ppm. They are formed with a typical size of about 0.23 \(\upmu\)m and a comparatively high yield as confirmed by DLS measurements (Figure 1a) and by the SEM images (Figure 1b). A reduction of the particle size is observed and shorter dendritic branches are formed if the poly-DADMAC concentration is slightly lowered (Figure 1c,d). A further reduction of the polycation concentration results in aggregate particles with a size of about 80 nm, which are obviously composed of elementary particles of about 10 nm in diameter (DLS spectrum and SEM image in Figure 1e,f). The DLS spectra show an increase in the mean particle size with the increase in poly-DADMAC concentration up to 400 ppm (Figure 2). It has to be reconsidered that the DLS measurements supply only a rough estimation for the size of larger nonspherical aggregates consisting of particle clusters and macromolecular filaments. Zeta potentials in the positive range (about +30 to +50 mV) have been observed in all cases of particle preparation in the presence of PDADMAC, as expected (Figure 2). Figure 1: Dynamic Light Scattering (DLS) spectra and scanning electron microscope (SEM) images of gold nanoparticles formed in the presence of poly-DADMAC at 50 \(\upmu\)M HAuCl\({}_{4}\) and 100 \(\upmu\)M ascorbic acid in aqueous solution: **(a,b)** 8 ppm poly-DADMAC, **(c,d)** 15 ppm poly-DADMAC, **(e,f)** 30 ppm poly-DADMAC and **(g,h)** 60 ppm poly-DADMAC. compact gold nanoparticles between 510 and 600 nm arises after the synthesis at lower poly-DADMAC concentrations. The peak wavelength is only moderately redshifted (to about 560 nm) in comparison with the well-known resonance of small spherical gold nanoparticles at about 530 nm if poly-DADMAC is added at a low concentration (8 ppm). The increase in poly-DADMAC concentration causes a bathochromic shift of the peak maximum and a strong increase in the absorption in the near-infrared (NIR) range. The optical spectra show the appearance of a broad band above 800 nm with increasing poly-DADMAC content. It is assumed that this increase is due to the electronic interaction of metallic nanoparticles coming in contact with each other by the aggregation process caused by the macromolecules (Figure 3). The strong increase in NIR absorption can be attributed to the formation of the dendritic structure of the formed particles. It corresponds to the long-wavelength absorption of nonspherical gold nanoparticles--nanorods, platelets or complex crystal structures--with higher aspect ratios [43; 44; 45]. Most DLS measurements indicate the formation of particles with an apparent diameter greater than two microns and indicate the formation of particles and clusters in the mid-nanometer and submicron range as well. The formation of larger aggregates was confirmed by SEM images (Figure 4) and by light microscopy. Typically, the aggregates show a filamentous morphology and consist of many single particles.