## 2 Materials and Methods  
Ascorbic acid (Alfa Aesar; 99+%), poly(diallyldimethylammonium chloride) solution (ALDRICH, molecular weight \(\leq\)100 kDa, 35 wt.% in water, PCode: 100103420), poly(sodium 4-styrenesulfonate) (ALDRICH, 1000 kDa, PCode: 1002572468) and tetrachloroauric acid trihydrate, HAuCl\({}_{4}\) (ROTH, p.a.), were used as received without further purification. All solutions were set up with water from a reverse osmosis plant.  
The particles were synthesized in agitated microreaction tubes by mixing appropriate amounts of 0.1 wt.% and 0.01 wt.% aqueous poly-DADMAC solutions, 1 mM HAuCl\({}_{4}\) and water to a final reaction volume of 1 mL and initiating the reaction by adding 2 mM ascorbic acid. Concentrations given with the samples are always final concentrations in the reaction dispersion. The synthesis experiments were carried out at room temperature (20 \({}^{\circ}\)C). For the formation of assembly particles, firstly, the polymer solution was diluted with water. The mixing ratio was chosen to achieve a final volume of 1 mL in each experiment. Secondly, the reducing agent (ascorbic acid) was added and mixed by vigorous vortexingfor about 20 s. In the last step, the metal precursor (HAuCl4) was added in a concentration of 1 mM. The total ion concentration was in the submillimolar or micromolar range in the final reaction mixture, typically. No additional salt was added. The ionic strength was given by the ionic reactants PDADMAC and [HAuCl4], which was in the range up to 400 ppm (PDADMAC) and 0.2 mM ([HAuCl4]). Additional chloride ions were generated by the reaction of tetrachloroaurate. Therefore, it was expected that the poly-DADMAC would be highly ionized under the applied experimental conditions.  
The sedimentation of particles was comparatively slow. In a standard UV/VIS spectrometric cuvette (1 cm), beginning sedimentation was proved after about 5 h (Supplementary Figure S1). The particles were stable and could be redispersed by support of ultrasound.  
Dynamic Light Scattering (DLS) measurements were conducted with a Zetasizer Nano-ZS (Malvern Panalytical). For the DLS measurements (25 \({}^{\circ}\)C, aqueous solution, absorbance: 0.3, viscosity: 0.8872 cP, equilibration time: 120 s, measurement angle: 173\({}^{\circ}\), backscattering mode, three single measurements per sample, 70 size classes between 1 and 2500 nm), 60 \(\upmu\)L of the as-obtained samples were transferred to a single-use cuvette and the cuvette capped off. The DLS measurements were performed 4 days after particle preparation and a redispersion (3 min shaking with ultrasound). In the DLS measurements, PDIs in the range between about 0.1 and 0.5 were found. Details are given in the supplementary table. SEM images were obtained in a Hitachi S-4800 FESEM (secondary electron signal, acceleration voltage: 25 kV) and optical spectra were measured in a SPECORD 200 (Analytik Jena). For the SEM measurements, the as-obtained samples were drop-casted onto silicon chips, dried in air and washed with deionized water.  
From the point of view of characterization of nanoparticle assemblies, a combination of spectrophotometry, DLS measurements and contrast-enhanced SEM imaging (measurement by secondary electron detector) was chosen. These three methods address very different aspects of the formation of particle aggregates. Spectrophotometry supplies information about the electronic states, the so-called particle plasmonic resonances and their change due to electronic coupling in the case of particle contact. DLS measurements supply information about particle mobility (Brownian motion), which is related to the mass and shape of particles and the properties of the embedding medium (solution, swelling). Finally, the contrast-enhanced SEM measurements allow the combining of signals from optimal imaging of materials with a high yield of secondary electrons and low transparency (metals) with signals from optimal imaging of materials with a low yield of secondary electrons and high electron beam transparency.