## 3 Results and Discussion
The sizes of these filamentous aggregates are mainly between about two and six microns. The preferential appearance of clusters in this size range and the rough estimation of size ranges in solution by DLS measurements indicate that the clusters are already present in the solution and not formed during the drying process in the preparation of chips for SEM imaging. Contrast-enhanced SEM imaging of native deposited colloids on polished silicon chip surfaces--without any additional deposition of conductive material for SEM sample preparation--shows that larger aggregates of metal nanoparticles are formed in which the connections between metal particles are due to small molecular filaments. These connecting filaments become visible in image processing under strong contrast enhancement (Figure 5). The length of these filaments is in the order of magnitude of one or a few microns. Their diameter can be estimated to be below 20 nm. The structure of the filaments and the low contrast in SEM  
Figure 3: Bathochromic shift of absorption maximum and increase in near-infrared absorption with increasing poly-DADMAC concentration during the formation of gold nanoparticles by the reduction of 50 μM HAuCl4 with 100 μM ascorbic acid. imaging indicate the presence of macromolecules. In the absence of other reactants, the assumption is that the filaments are mainly formed by poly-DADMAC. The incorporation of gold nanoparticles inside the aggregates is plausible because they are negatively charged after their formation by the reduction of tetrachloroauric acid by ascorbic acid. Thus, the large aggregates are probably formed due to electrostatic binding between the negatively charged metal nanoparticles and the positively polycationic macromolecules. On the one hand, the poly-DADMAC molecules glue the nanoparticles to each other; on the other hand, the metal particles act as knots in the macromolecular network. There is likely a competition taking place between solvent and nanoparticle interactions with the ammonium groups. This effect, the mobility of polymer chains and repulsive forces between free ammonium groups lead to the formation of the observed noncompact aggregates in the lower micrometer range, as observed in the SEM images. Figure 4: Formation of larger filamentous gold nanoparticle aggregates by the reduction of HAuCl4 (50 uM) with ascorbic acid (100 uM) in the presence of poly-DADMAC (5 ppm): (**a**) SEM image showing several aggregates, (**b**) SEM imaging showing one filamentous aggregate at enhanced magnification. Figure 5: SEM images (overlay of contrast-enhanced images) of the aggregation of gold nanoparticles formed by the reduction of HAuCl4 (20 uM) with ascorbic acid (60 uM) in the presence of poly-DADMAC (100 ppm): (**a**) larger cluster of metal nanoparticles; (**b**) macromolecular filaments. The present nanoparticle structures suggest the formation of a hierarchical organization of gold nanoparticles in the aggregates. Therefore, four levels of structural complexity could be identified (Figure 6): The single compact gold nanoparticles represent the first level. These gold nanoparticles can be small compact nanocrystals or even nearly spherical polynuclear gold nanoparticles which are formed by a nucleation/assembling mechanism, as discovered by [PERSON] et al. [46]. The typical size of these particles is in the order of magnitude of 10 nm. The second level is marked by aggregates of these particles found in the form of compact particle clusters (see, e.g., Figure 1f) with typical diameters in the range between about 20 and 100 nm. The third level of particle assembling supplies aggregates with diameters in the submicron range. It can be found either in the form of composed dendritic particles with a granular surface (Figure 1b) or in the form of aggregates of smaller particle clusters as shown in Figure 1h. These particles have the character of superclusters because they consist of smaller particles and particle clusters. Finally, the fourth level of assembling is related to aggregates of the superclusters.