please refer about Prolindac(this is the HPMA polymer conjugate of Pt complexes clinical trials). HPMA, BPEI, PEG, Chitosin, and bio-compatible polymers are well known polymers for delivery. You should check the breast cancer cells properties and your polymer nature (toxicity varies the factors the molecular weights, the branched and linear) some research groups are using  co-polymers for drug delivery. which one is opt. My suggestion cationic polymers. because cheaper and very easy synthetic procedure. Thank you

You need to search which biological markers are more expressed in breast cancer cells. For example, most cancers express a higher level of folate receptors, so if your attach folate to your polymer it will target more cancer cells than healthy cells. The downside of that approach is that it's not specific for breast cancer nor only cancer cells, it only increases your chances.

Just search for breast cancer hallmarks and biomarkers.

Hope I could help.

If you are using inorganic porous material like mesoporous silica and planning to target to breast cancer then it will be a tedious process owing to absence of multifuntional groups on silica.

In line with your question, I would suggest you to first plan your strategy for delivering the nanoparticles to tumor. Like if you just need to deliver drug loaded nanoparticles to breast cancer then you can select Her 2 antibody or VEGF antibody or simply folic acid and targeting ligand. But apart from these you also need to see if these nanoparticles are outside cell or inside cells. Like the targeting ligand will increase concentration of your nanoparticles around tumor cell but you need to enhance the cellular uptake of nanoparticles inside breast tumor cells. So, for that you can go for attachment of cell penetrating peptide i.e CPP. 7R (RRRRRRR) is the most recently used CPP. 

I hope this will help you.