ESC command printing android bold

I have an android app that prints via bluetooth but I have an issue sending an ESC command for a bold character.

StringBuilder builder = new StringBuilder();
builder.append("Data Ordine: " + mDetailsOrderList.get(0).date_add + "\n");
builder.append("\x1B\x21\x08"+"Riferimento: " + "\x1B\x21\x00" + mDetailsOrderList.get(0).reference + "\n");

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\x1B\x21\x08 able bold ESC ! 08
\x1B\x21\x00 diable ESC ! 00

For example, I want riferimento as bold but don't accept this syntax..illegal character "/"

Could you help me solve this issue please?