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It is becoming increasingly apparent that small molecules can self-assemble into a wide-range of nano-entities in solution that have intriguing properties. The recently introduced NMR aggregation assay is playing an important role in revealing these nano-entities. Here, we employ the NMR aggregation assay to expose the self-aggregation tendencies of dyes in solution. This dilution-based assay demonstrates that some dyes can exist as single-molecule entities whereas others can adopt aggregates of distinct sizes. Interestingly, dyes with highly related chemical structures can adopt largely different sized aggregates - demonstrating the existence of structure-nanoentity relationships (SNR) – which suggests that they can assume and/or be designed to have distinct properties. One property was evaluated where the drug Quetiapine (Seroquel) was added to the dye Congo red which resulted in the absorption of the drug into the dye nano-entity. This showed a direct drug-dye interaction, and it demonstrated that dye aggregates can have influences on drug solution behaviors. The NMR method described in this study provides a practical and valuable tool to monitor dye aggregates and to better understand their associated properties (e.g. toxicity, off-target activity) and potential utility (e.g. drug encapsulation, drug delivery systems). [ABSTRACT FROM AUTHOR]