Impact of nanoparticle size and surface chemistry on peptoid self-assembly

Self-assembled organic nanomaterials can be generated by bottom-up assembly pathways where the structure is controlled by the organic sequence and altered using pH, temperature, and solvation. In contrast, self-assembled structures based on inorganic nanoparticles typically rely on physical packing and drying effects to achieve uniform superlattices. By combining these two chemistries to access inorganic-organic nanostructures, we aim to understand the key factors that govern the assembly pathway and structural outcomes in hybrid systems. In this work, we outline two assembly regimes between quantum dots (QDs) and reversibly binding peptoids. These regimes can be accessed by changing the solubility and size of the hybrid (peptoid-QD) monomer unit. The hybrid monomers are prepared via ligand exchange, assembled, and resulting assemblies are studied using ex-situ transmission electron microscopy as a function of assembly time. In aqueous conditions, QDs were found to stabilize certain morphologies of peptoid intermediates and generate a unique final product consisting of multilayers of small peptoid sheets linked by QDs. The QDs were also seen to facilitate or inhibit assembly in organic solvents based on the relative hydrophobicity of the surface ligands, which ultimately dictated the solubility of the hybrid monomer unit. Increasing the size of the QDs led to large hybrid sheets with regions of highly ordered square-packed QDs. A second, smaller QD species can also be integrated to create binary hybrid lattices. These results create a set of design principles for controlling the structure and structural evolution of hybrid peptoid-QD assemblies and contribute to the predictive synthesis of complex hybrid matter.

reversible thiol-maleimide click reaction that generates a covalent linkage between the nanoparticles and the assembled peptoid macrostructure. This approach allowed for control over the nanoparticle density as a function of the peptoid side-chain composition. Inorganic species have also been directly appended to the hydrophilic end of the peptoid monomers during the initial solid-phase synthesis, creating a hybrid peptoid monomer that can, in principle, then be assembled into larger-scale structures 39 . In this work, we demonstrate the versatility of a related approach by using peptoid monomers as ligands on quantum dots (QDs) to tune assembly pathways bottom up, with the final hybrid structures being a function of QD concentration, surface chemistry, and size.
CdSe and CdS QDs were attached to peptoid monomers to study the impact of these different pendant groups on assembly. The peptoid monomers contained six hydrophobic residues (N- [2-(4-bromophenyl)ethyl]glycine, Nbrpe6Dig) and were terminated with a carboxylic acid (Scheme 1). This relatively short chain peptoid was chosen for its structural simplicity, diverse solubility properties, and limited steric bulk. A carboxylic acid group was chosen as the binding moiety due to the reversibility of the interaction with the QD surface, allowing for attachment through a facile carboxylic acid -carboxylate ligand exchange reaction 45 . Furthermore, the dynamic interaction between the QD and the peptoid monomer provides a mechanism for the system to reach equilibrium and reduce the heterogeneity in the number of peptoids bound to a given QD. By starting with QDs capped with carboxylate ligands, we estimate an approximate equilibrium constant for the ligand exchange with peptoid of one 45 . For the QD composition, CdSe and CdS were chosen due to the depth of existing literature for these materials in diverse solvents, as well as the well-developed knowledge on ligand exchange and binding affinities for many different systems [45][46][47][48] . Cadmium chalcogenides are known to be relatively stable in water without significant surface modification 46 . Using QDs as pendant groups on the peptoids allows for straightforward modification of the peptoids without drastically modifying the synthetic chemistry or reactivity. This allows for a systematic study into how the pendant groups' size, hydrophobicity, and concentration alter the assembly process. This method further allows for a single peptoid sequence to provide access to a variety of structures without having to design a new sequence and introduces driving forces for assembly not accessible with peptoids alone. Scheme 1. General conditions for carboxylic acid (red)-terminated peptoid (blue) ligand exchange onto QDs (yellow) and assembly conditions for hybrid sheet formation.

Results and Discussion.
Peptoid and hybrid material assembly as a function of the peptoid monomer:QD ratio.
To study assembly in a range of solvents, CdSe QDs (2.4 nm) were synthesized with 2-[2-(2-methoxyethoxy)ethoxy)acetic acid (MEEAA) to generate QDs with a wide solubility range in both aqueous and organic solutions. Peptoid monomers were added to the QDs in varying concentrations (0.3-20 eq. of peptoid monomers per QD) and incubated for 18 h in MeCN. Incubation for 18 h allowed the system to reach equilibrium and resulted in a partial ligand exchange, as evidenced by 1 H NMR ( Figure S1). After 18 h, water was added to the solution to initiate the assembly process. This solution was allowed to assemble at 4 °C for up to 4 weeks via slow evaporation of MeCN. Ex situ transmission electron microscopy (TEM) was the primary characterization mode, with scanning TEM (STEM) and bright field (BF) to identify the microstructures and nanostructures, respectively. The assembly process was tracked by taking aliquots from the reaction solution and preparing TEM grids. Initial samples were taken prior to the addition of water (0 h) to determine if any assembly occurred without anti-solvent addition. Image timepoints for the assemblies were taken relative to anti-solvent addition at 1 h, 24 h, 4-5 d, 2 w, and 4 w.
Peptoid assembly was first tracked in the absence of QDs to gain a baseline for comparing intermediates arising during the assembly process. The peptoid partially dissolved in MeCN to produce non-uniform, elongated sheets and hollow spheres ( Figure S2) that evolved into large, aggregated sheets after 18 h (Figure 1a). Water addition solubilized the peptoid, facilitating rearrangement into small sheets and micelles (Figure 1b,c).The spherical structures were assigned as micelles due to their solid nature and small size (20.9 ± 4.1 nm). Over the course of 4 weeks, the micelles remained present while sheets continued to grow until rectangular sheets with straight edges were observed (Figure 1e).
Adding MEEAA-CdSe QDs into the peptoid solution prior to assembly produced a different final product and altered the nature of the intermediates. In MeCN, peptoid fibrils with QDs on the edges were observed rather than large peptoid-based aggregates (Figure 1f,k). With the addition of water, the nanofibers remained in addition to peptoid micelles with QDs on the edges (Figure 1g,l). Upon initial imaging, the peptoid-QD spheres appeared solid in STEM mode, but prolonged imaging led to hollow structures with high contrast exteriors ( Figure S3a). The production of hollow structures implies a carbon-rich center that was destroyed by the electron beam. BF mode confirmed QDs only present on the edges, thus the assignment of micelle-like peptoid structure with QDs adhering to the surface (Figure S3b,c). The CdSe QDs were seen to etch upon exposure to the peptoids, resulting in a diameter of 1.9 ± 0.2 nm that remained constant for up to 4 weeks in solution ( Figure S4). After 24 h, the QD-decorated micelles were seen to arrange into a linear pattern with an average spacing of 89.4 ± 51.6 nm (Figure 1h). The distance between hybrid micelles was too large to correspond to one or two peptoid monomers, therefore we hypothesize a thin peptoid ribbon or fiber connecting the structures that was not easily seen next to the high contrast QDs (Figure 1m). Over the course of 4 weeks, the connected micelles began to ripen through bending of the linear structures ( Figure  S5) to form small, QD-covered sheets (Figure 1i,n), reminiscent of structures formed upon addition of QDs to pre-formed sheets 41 . These small hybrid peptoid/QD sheets eventually aggregated to form large multilayer aggregates (>5 m, Figure 1j,o). This final hybrid multilayer material contained small QD-decorated peptoid sheets similar to those observed earlier in the assembly, but with a higher density of QDs on the edges ( Figure S6). We propose that these QDs act as linkers to bind the small sheets together into one large assembly, as has been previously observed for silica-peptoid hybrid structures 42 . This series of experiments demonstrate the synthesis of hybrid materials in which both the QDs and the peptoid operate synergistically to dictate the final structure. Peptoid micelles (c) are seen throughout the assembly process as the sheets grow to be welldefined (d). After 4 weeks, large crystalline sheets are seen (e). Peptoid edges are denoted by white dashed lines. (f-o) Assembly over time of MEEAA-CdSe QDs with 1 eq. of peptoid in H2O/MeCN. Peptoid fibrils with QDs on the edges are seen before and right after water addition (f,g white arrows). Peptoid micelles with QDs on the edges are also observed (g) that tend towards linear arrangements (h). After 5 days, small peptoid sheets are seen with a uniform QD distribution (i). Finally, the QDs are seen to concentrate at the sheet edges and facilitate the formation of large multilayer aggregates (j) composed of many small sheets (red inset, j). Peptoid edges are denoted with white dashed lines. Schematic representation of the assembly process (ko) with QDs in yellow, peptoid backbone in blue, and peptoid carboxylic acid in red.
The importance of the peptoid in this system was investigated by changing the number of peptoid monomers relative to the CdSe QD and changing the sequence of peptoid used. 20-0.33 equivalents of peptoid relative to QD were investigated while keeping the peptoid concentration constant to allow for direct comparison of assemblies at a given time point. At high peptoid equivalents, the system was seen to phase segregate with minimal integration of QDs into the structure. Here we propose the QDs are acting as defects, causing rolling and elongation of the sheets after assembly ( Figure S7a). Low peptoid equivalents (5-0.33 eq.) led to assembly pathways with the same intermediates and end products as the one equivalent case described above (Figure S7b-e). However, the speed of the assembly and the stage at which different intermediates appeared varied in a consistent manner based on peptoid equivalents ( Figure S8).
In general, we observed a decrease in the overall rate of progression at higher peptoid equivalents. We hypothesize this decrease in assembly speed is due to a decrease in diffusion of the hybrid peptoid-QD monomers as the number of peptoids on the surface of the QDs and the size of the hybrid unit increases.
Changing the sequence of the peptoid to an asymmetric sequence with alternating (4bromophenyl)ethyl and (4-bromophenyl)methyl side chain groups, (NbrpeNbrpm)3Dig ( Figure  S9a), which forms twisted nanoribbons in absence of QDs, yielded inconsistent structures both within a given TEM grid and across samples regardless of timepoint or peptoid equivalents ( Figure S9). However, alteration of the assembly pathway with QDs was seen and gave structures distinct from the peptoid alone. The assemblies showed very large hybrid materials as early as 1 h for all samples but did not show any distinguishable trends. It is hypothesized that the asymmetric sequence forms a twisted structure due to the differences in packing between the two sides of the peptoid 31,32,36,37 . By having a bulky QD at the end of the sequence, this difference in packing could be disrupted and the extent of the disruption may depend on how many peptoids are on a given QD, yielding inconsistent products. As a result, the rest of the studies described below were performed with the symmetric peptoid (Nbrpe6Dig) at 1 eq. per QD.
Modulating QD surface chemistry to improve assembly in organic solvents.
By changing the ligand on the QD surface we can directly probe how the hydrophobicity of the peptoid-QD unit impacts the assembly process, allowing us to determine how different design elements lead to changes in the reaction landscape and ultimately the structure of the final assemblies. CdSe QDs (2.4 nm) were synthesized with oleic acid (OA) and MEEAA to generate hydrophobic and amphiphilic QDs, respectively, while maintaining carboxylate binding groups. To better solubilize the OA-CdSe QDs, the mixtures were assembled in dimethyl sulfoxide (DMSO) and chloroform (CHCl3). Chloroform was chosen to dissolve the OA-CdSe QDs and peptoid monomers, replacing acetonitrile as the evaporating solvent, and DMSO to replace water as the higher boiling point polar solvent to create unfavorable interactions with the aryl halogens on the peptoid. The peptoid monomer was assembled alone in these solvents to get a baseline for peptoid assembly. In CHCl3, the peptoids formed a mixture of large fibers and small rectangular sheets immediately after dissolution ( Figure S10). After 18 h, only aggregates of large hollow spheres several hundred nanometers in diameter were present (Figure 2a). With the addition of DMSO, the structures collapsed and shrank until peptoid fibers with spheres attached to the fiber edges formed at 5 days (Figure 2b,c). Also present at 5 days were small, amorphous sheets. The fibers and sheets grew slightly over the course of a few weeks but never formed crystalline sheets, as evidenced by their irregularity in size, shape, and contrast in TEM (Figure 2d). Given that assembled structures were present immediately after dissolution, we believe the observed structural evolution is based on peptoid monomer rearrangement rather than complete dissolution and re-assembly from monomers, as seen in aqueous conditions. In general, these observations suggest hindered assembly in the organic solvent system, likely due to decreased solubility of the monomer units. . Peptoid alone showed large hollow spheres (a) that collapsed to form large aggregates (b), and eventually fibers (white dashed line, c) decorated by small dense spheres (arrow, c). Some sheet formation was observed (yellow dashed line) but no large, well-defined sheets were observed (d). The presence of MEEAA-CdSe QDs produced similar large hollow spheres decorated by QDs prior to DMSO addition (e), but then facilitated the solubilization of the peptoid, generating sheets with an even distribution of QDs(f). Over time QDs concentrated at the edges (g), ultimately resulting in assembly of the small sheets into large multilayer hybrid sheets similar to what was observed in MeCN/H2O (h). OA-CdSe generated a hydrophobic hybrid monomer unit that formed large 2D sheets in CHCl3 (i). Upon addition of DMSO collapse to form small 2D aggregates (j) that evolved into 3D hybrid multilayers (k), and eventually phase segregation of peptoid (white arrows) and QD (l).
Adding QDs with high coverage of an amphiphilic ligand should increase the solubility of the hybrid monomer unit and facilitate the formation of sheets. MEEAA-CdSe was assembled with 1 eq. of peptoid using the organic solvent system. Prior to the addition of DMSO, the QDs did not alter the hollow peptoid structures, but were instead bound to the exterior (Figure 2e). After the addition of DMSO, small peptoid sheets evenly decorated with QDs rapidly appeared and underwent aggregation over time (Figure 2f,g). The hybrid structures formed over 4 weeks primarily included large assemblies composed of smaller sheets with a higher density of QDs on the edges (Figure 2h), similar to the assemblies observed in MeCN/H2O. The assembly of hybrid monomer units in organic solvents did not produce spherical and fiber intermediates as in MeCN/H2O (Figure 1f-h) but proceeded directly to hybrid peptoid-QD sheets after only 1 h. The QDs were again seen to etch to 1.72 ± 0.22 nm, as was observed in the aqueous system. This more rapid progression to the hybrid sheet structures suggests that the QD-peptoid monomer unit is more soluble than the peptoid alone in the organic solvents, resulting in greater lability and faster assembly. Interestingly, these observations are opposite to the trends observed in the aqueous system where additional intermediates and slower sheet formation were noted in the case of the QDs. Together these data suggest that monomer solubility plays a crucial role in the assembly dynamics and that this solubility can be dramatically tuned based on the ligation of the appended QDs.
To further push the assembly pathway away from evaporative ordering based on hydrophobicity, OA-CdSe QDs were assembled with 1 eq. of peptoid. A hybrid unit with OA-CdSe yields a highly hydrophobic unit with both the QD surface and the exposed portion of the peptoid being insoluble in DMSO. Prior to DMSO addition, large 2D hybrid sheets were seen after 18 h in CHCl3 with lengths greater than 15 m (Figure 2i). Within the sheets there were areas of folding or curling, but they appeared to be primarily a monolayer of QDs without any distinct ordering among the QDs (Figure S11). This sheet formation indicates that by changing the QD surface chemistry, assembly can be induced without the addition of an antisolvent and without any evaporative processes as a driving force. With the addition of DMSO, the sheets collapsed initially into 2D circular structures that grew into larger 3D aggregates over time (Figure 2j,k). We hypothesize that this 3D aggregation is due to the frustrated structure where neither the QDs nor peptoids are sufficiently soluble in the CHCl3/DMSO solvent mixture, resulting in amorphous packing of the hydrophobic units. After most of the CHCl3 evaporated, phase segregation was observed between the peptoids and QDs with large peptoid-rich structures forming and free QDs scattered across the TEM grid (Figure 2l). This phase segregation suggests that the driving force for peptoids to assemble is still present under these conditions, but the peptoids are unable to order in a favorable manner with the QDs attached.

QD-mediated synthesis of hybrid structures and binary-QD lattices.
Another route to change the peptoid-QD assembly is by changing the size of the QD relative to the peptoid and increasing the overall bulk of the hybrid monomer unit. CdS QDs were synthesized with OA capping ligands and then were subjected to ligand exchange to produce 5.1 nm MEEAA-CdS QDs. These larger QDs have similar reactivity and surface chemistry to the MEEAA-capped CdSe QDs but have much more distinct and well-defined surface facets due to the increase in size. The same general CHCl3/DMSO assembly process was followed as above with one peptoid monomer per QD, with the only notable difference being the overall volume of QD was much larger than previously due to the increased QD size. In CHCl3, large, amorphous peptoid aggregates were seen with QDs on the edges (Figure S12a). QDs were seen to order into a linear packing pattern near the edges of the peptoid aggregates ( Figure  S12b,c). Exposure to the peptoids led to an initial etching of the QDs to give a diameter of 3.50 ± 0.37 nm. These peptoid aggregates with linear ordered QDs on the edges were maintained after DMSO addition with the appearance of large spherical aggregates comprised of primarily QDs, likely due to the high QD coverage (Figure 3a). By 24 h, large 2D sheets were seen on the order of 10 m that were persistent through the rest of the evaporation process (Figure 3b). Closer inspection of the sheets revealed the QDs packing in a square close packed array, with one QD having four nearest neighbors (Figure 3c). This ordering was observed in patches throughout the sheets with crystalline domain sizes ranging from 15 to 100 nm ( Figure S13). These sheets were noticeably distinct from other assemblies we had observed previously and are reminiscent of structures that are dictated by inter-QD interactions arising from oriented attachment (vide infra), with the peptoid acting as a surface modifier [49][50][51][52] . This indicates that having a large pendant group relative to length of the peptoid disrupts the expected inter-peptoid interdigitation, preventing the formation of standard peptoid sheets. Ordered nanoparticle superlattices with multiple types of nanoparticles have been explored through slow evaporation on a surface or at an interface, as well as by functionalization with ligands with chemical complementarity like DNA 11,19,53,54 . In the systems based on evaporation, the nanoparticle packing can be altered by changing the sizes of the nanoparticles relative to each other and their relative stoichiometries 54 . We sought to combine the CdSe and CdS QD systems to access free-standing binary QD assemblies. A total of 1 eq. of peptoid per total QD was combined with CdSe and CdS (1:1) and assembled in CHCl3 and DMSO. The general assembly pathway was consistent with the CdS system, but sheet generation was slightly slowed. Peptoid micelles, previously seen in the CdSe system, were also observed at early time points with QDs in a linear pattern near the edges (Figure 3d). The QDs were dominated by CdS, with CdSe being relatively difficult to identify, but were observed throughout the time series with the identification of ~2 nm particles (Figure S14). We propose that this dominating effect of the CdS is a result of the more well-defined faceting and greater total volume of CdS present compared to CdSe. Over the course of 4 weeks, the sheets grew while remaining 2D until at 4 weeks ordering was observed within the sheets in a similar square close packed pattern as seen with CdS QDs alone (Figure 3e,f). When CdS and CdSe were mixed without peptoid present, there was no evidence of ordering between the QDs (Figure S15). Some aggregation of the QDs was seen due to high QD concentration but no free standing, 2D structures were observed in CHCl3 or CHCl3/DMSO. Increasing the ratio of CdSe to CdS did not lead to different packing structures, but rather a slow transformation of large sheets to smaller peptoid-QD sheets attached via bridging QDs. With increasing CdSe:CdS, the packing became more disordered, moving from square-packed QDs to linear packing, until total disorder was observed ( Figure S16). These results suggest the CdSe acts as an "impurity" in the system, not dictating the structure as the CdS does. As the concentration of impurity increases, the sheets begin to break apart into smaller structures and aggregate in a fashion similar to that seen in the CdSe only system. Assembly of CdS and CdSe QDs into ordered arrays did not show significant changes in the optical properties. No change was observed in the location or intensity of the photoluminescence for the assemblies and a 2 nm redshift was seen for the absorbance ( Figure  S17). We hypothesize that the lack of long-range order and decreased solubility of the assemblies with time give properties comparable to free QDs.

Characterization of CdS and CdS/CdSe ordered arrays and the role of peptoid.
To understand the formation of these ordered CdS and CdS/CdSe assemblies further, we sought to characterize their structure and dependence on drying through TEM image analysis, atomic force microscopy (AFM), and altering sample deposition. The porous spacing between the CdS particles was seen to be 1.75 ± 0.20 nm via TEM (Figure 4a). For 3.5 nm particles (after etching), without accounting for ligands present, the pore spacing between particles would be expected to have a diameter of 1.4 nm. Accounting for ligands on the QD surface that are ~1.5 nm in length with minimal interdigitation between neighboring ligand shells, a spacing of 1.9 nm would be expected (Figure 4b). The packing for mixed QDs showed a slight decrease in the pore size via TEM (1.64 ± 0.20 nm) compared to CdS QD assemblies (Figure 4e). Interestingly, an apparent necking feature was seen between the CdS QDs in the mixed QD samples with a width of 1.65 ± 0.25 nm, consistent with the diameter of the etched CdSe QDs (Figure 3f, yellow  dashed lines). This feature did not appear to be consistent with particle fusion due to discontinuity of the lattice fringes and its round instead of tapered appearance, but rather the presence of a separate CdSe sublattice. Identification of the CdSe QDs was confirmed using high-resolution TEM to show d spacing values of 0.35 nm and 0.23 nm, consistent with (111) and (220) planes of zinc-blende CdSe (Figure 4e). If the CdSe were positioned in the plane with the CdS, accounting for partial interdigitation of the ligand shells, we would expect an increase in pore size to 2.3 nm (Figure 4c). However, positioning the CdSe above or below the plane of CdS at interstitial sites results in the lattice of CdS being preserved with an apparent reduction in the pore size (1.6 nm) due to additional material at the interstitial sites (Figure 4d). Additional characterization of the assemblies was done via AFM to confirm the QD ordering and determine the position of CdSe relative to CdS. Due to the large size of the sheets, the edge of the sheet was not identified with AFM. Nevertheless, the surface of the sheet showed a patchy structure with ordering throughout (Figure 4f). The patches on the surface showed a layered structure with many layers exhibiting square close packed ordering ( Figure S18). We hypothesize that these assemblies are highly ordered in two dimensions but lack registry in the Z direction due to the overlap of the sheets. AFM images showed the CdSe are not in plane with the CdS as no small QDs were observed (Figure 4g-h, orange line). However, line scans of the ordered assemblies showed an intermediate peak between CdS, presumably from a layer of CdSe QDs offset from the top layer (Figure 4g-h, blue line).
Square close packing has been observed extensively for lead chalcogenide QDs both through fusion and drying effects due to decreased ligation on specific facets [49][50][51][52] . It has been shown that this fusion process occurs through oriented attachment under conditions in which ligand coverage on the (100) facet is reduced [49][50][51][52]55 . This oriented packing is not observed for QDs smaller than 4 nm as the (100) facet is not present in the smaller nanoparticles 52,55 . When the particles are not fused, these lattices arise due to drying effects. In our system, if the peptoids were acting as isolated ligands on the surface of the CdS QDs and only the QDs dictated the structure, one would expect the sheets to arise due to similar drying effects, not the observed solution-phase assembly. To confirm the sheets form in solution due to peptoid and QD contributions, the sheets were isolated by slow centrifugation of the solution, producing a yellow precipitate that was resuspended in DMSO. Large sheets formed in solution would be expected to settle out under these conditions with any free QDs remaining in solution. TEM analysis showed the same 2D sheets in the pellet with slight destabilization seen by a reduction in the percentage of ordered arrays (Figure S19). The supernatant only showed structures associated with random aggregation of free QDs and no evidence of ordering within the aggregates. These results indicate that large structures are present in solution and resemble the large sheets present in aliquots taken from solution. To further determine if the sheets are present in solution or result from a drying effect, the TEM grid sample preparation method was altered. To disrupt any drying effects due to slow evaporation, the sample was drop cast onto the grid and let settle for 2 min, dabbed off, and immediately placed under vacuum to rapidly dry the sample. In contrast, typical samples are allowed to settle for 15 min before dabbing off the grid and allowing to dry on a Kimwipe for an additional 15 min before exposure to the vacuum. Analysis of the rapidly dried grid still showed the same large 2D sheets with square close packed QDs, as seen previously ( Figure S20). Moreover, the prevalence of large QD aggregates was decreased and the sheets appeared thinner at the edges relative to standard sample preparation, implying that faster drying of the sample reduces overcrowding and stacking of free QDs on the grid.
Finally, to determine if the assemblies form because of bottom-up assembly and integration of QDs into the sheets or decoration of QDs onto a pre-formed peptoid surface, 1:1 CdS and CdSe QDs were added to pre-formed peptoid sheets in DMSO and allowed to incubate at room temperature. Upon initial addition, QDs were seen to stick non-uniformly to peptoid surfaces, with the sheets mostly remaining intact (Figure S21a). Over the course of 2 weeks, large sheets similar to bottom-up assembly were observed but did not show ordering of the QDs and stability was lost over time to produce small, rounded peptoid sheets with uneven QD distribution (Figure S21b-e). These results indicate that the hybrid sheets are not formed due to drying effects alone and the QD superlattices do not arise from post-synthetic coordination to assembled peptoid sheets. Instead, these results support a hybrid system wherein the QDs and peptoids assemble in solution together as a hybrid unit from an integrated structure. We hypothesize that the peptoids are bound to the QD surface, acting as a ligand rather than an assembled template. On the QD surface the peptoid monomers can act as a linker between dots or bind specific facets, leading to the observed square packed superlattices where the inter-QD interactions dictate the final structure.

Conclusions.
To access new hierarchical hybrid materials, we must first understand the design principles that dictate the evolution of structure in these dynamic systems and determine how different parameters can be used to tune the reaction landscape. In this work, we have outlined two assembly regimes between QDs and peptoids that reversibly bind to the QD surface. In the first regime, the hybrid structures are controlled by both the peptoid and QD, whereas in the second regime, the hybrid structures are dominated by inter-QD interactions. The assembly pathway and progression of intermediates for peptoid-QD hybrid structures were studied in aqueous and organic conditions. Attachment of the QD to the peptoid altered the solubility of the hybrid monomer unit aiding the assembly in organic conditions and stabilizing intermediates in the aqueous system. Design rules such as the ratio of peptoid monomer relative to QD, hydrophobicity of the QD, and size of the QD were explored. Changing the equivalents of the peptoid at low concentrations only changed the speed of assembly based on the diffusion